Your dog is home from his neuter surgery, and you’re wondering when the testosterone — and the marking, mounting, and roaming that come with it — will finally be gone. The short answer: testosterone starts dropping within a day or two and reaches very low or undetectable levels by about six weeks. Neutering (castration) is the surgical removal of a male dog’s testicles, the organs that make most of his testosterone. Because the source is gone, levels begin falling almost immediately — but the testosterone already circulating in his body takes several weeks to clear, so it isn’t gone the moment he wakes up. This guide walks through the exact timeline, the six-week window when he can still get a female pregnant, and what to do if the behaviors you hoped to fix don’t fade.

How Long Until a Dog’s Testosterone Is Gone After Neutering?

For most dogs, testosterone reaches very low or undetectable levels around six weeks after surgery. It’s a gradual decline, not an overnight switch. The testicles produce the large majority of a male dog’s testosterone, so removing them cuts off the supply — but the hormone already in his bloodstream has to be metabolized and cleared, and that takes time.

A small amount of testosterone is also made by the adrenal glands, which is why levels never hit absolute zero. That residual amount is tiny and not enough to drive the hormone-fueled behaviors most owners are trying to reduce. So when people ask how long after neutering the testosterone is “gone,” six weeks to functionally gone is the honest answer, with a trace remaining from a non-testicular source.

The Testosterone Decline Timeline After Neutering

Testosterone doesn’t fall off a cliff the instant the testicles are removed; it tapers over about six weeks as the body clears what’s left.

How fast your individual dog moves through this depends on his age, size, and metabolism. A younger dog may clear it slightly differently than an older one, and a dog neutered later in life after years of high testosterone — may have more deeply ingrained behaviors even once the hormone is gone. The hormone timeline is fairly consistent; the behavior timeline is where dogs vary most.

Your Dog Is Still Fertile for About 6 Weeks

This is the part owners most often miss, and it matters: a freshly neutered dog can still get a female pregnant for several weeks after surgery. Because testosterone — and viable sperm that were already produced can linger during the clearance window, your dog remains fertile until his system fully clears, generally up to about six weeks.

The practical rule is simple: keep a newly neutered male away from intact (unspayed) females for a full six weeks. Letting them mingle too soon is a genuine and avoidable way to end up with an unexpected litter. If your dog has been around an in-heat female during this window, mention it to your veterinarian.

When Will the Behavior Change After Neutering?

Behavior lags the hormone. Even though testosterone is mostly cleared by six weeks, the behavioral effects take longer to settle — often 3 to 6 months — and some early shifts can show up sooner, around 2 to 4 weeks.

The behaviors most likely to ease as testosterone falls are the ones the hormone drives directly:

• Roaming and the urge to escape in search of females
• Urine marking
• Mounting and humping
• Some forms of testosterone-related aggression toward other male dogs

A fair, honest expectation: neutering often reduces these behaviors, but it doesn’t reliably “calm a dog down” in a general sense. It won’t fix hyperactivity, anxiety, or excitement-based behavior, because those aren’t driven by testosterone. A dog’s core temperament and energy level stay largely the same — what changes is the hormone-fueled subset of behaviors.

Why Is My Neutered Dog Still Humping or Marking?

This is the most common worry, and the timing tells you most of what you need to know. If it’s been less than six weeks, leftover testosterone is the likely driver — give it time. If your dog is still humping, marking, or mounting well after the six-week mark, the hormone is no longer the explanation, and two other causes usually are.

The behavior may be learned. A dog who humped or marked for months or years before surgery built a habit, and a habit isn’t a hormone — surgery alone won’t erase it. That requires training and consistent redirection, not more waiting. Or the behavior may not be hormonal at all. Veterinary behaviorist Dr. Karen Sueda notes that mounting is often a “displacement behavior” — something a dog does when he’s excited or anxious, or because it reliably gets him attention. A mounting habit that appears suddenly and frequently can even signal a medical problem, such as a urinary tract infection, which is worth a vet visit to rule out.

Managing the First 6 Weeks After Neutering

A little planning makes the clearance window smoother:

• Keep him away from intact females for the full six weeks to prevent an accidental litter.
• Manage your expectations. Hormone-driven behaviors fade gradually over weeks to months, not overnight. Judge progress at the 3-month mark, not day three.
• Protect the incision. Limit vigorous activity and prevent licking (an e-collar helps) until your vet clears him, usually within a couple of weeks.
• Watch for trouble. Call your veterinarian promptly for extreme lethargy, vomiting, bleeding or significant swelling at the incision, signs of pain like hiding or shaking, or pale gums.

If a behavior persists past six weeks, that’s your signal it’s no longer about hormones — and that’s a fixable situation with the right help.

The Bottom Line

If your dog is past the six-week mark and still humping, marking, or showing aggression — or if a mounting habit has appeared suddenly — call your veterinarian. Ask them to rule out a medical cause like a urinary tract infection and, if the behavior is a learned habit, to refer you to a certified dog trainer or veterinary behaviorist who can address it directly.

Frequently Asked Questions

How long after neutering is testosterone gone in a dog?

Testosterone begins falling within 24 to 72 hours and reaches very low or undetectable levels by about six weeks. A trace remains from the adrenal glands, but not enough to drive hormone-related behaviors.

Can a neutered dog still get a female pregnant?

Yes, for up to about six weeks after surgery. Residual testosterone and previously produced sperm can linger, so keep a newly neutered male away from intact females during that window.

Why is my dog still humping after being neutered?

Within six weeks, leftover testosterone is the likely cause. After six weeks, it’s usually a learned habit that needs training, a displacement behavior tied to excitement or anxiety, or occasionally a medical issue like a urinary tract infection.

How long until my dog’s behavior changes after neutering?

Some shifts appear within 2 to 4 weeks, but full behavioral changes typically take 3 to 6 months as hormone levels settle and habits adjust.

Do testosterone levels drop immediately after neutering?

They start dropping within a day or two, but they don’t vanish instantly. The hormone already in the bloodstream takes several weeks to clear, reaching very low levels by around six weeks.

Will neutering calm my dog down?

It often reduces hormone-driven behaviors like roaming, marking, and mounting, but it won’t change a dog’s core energy or temperament. Hyperactivity and anxiety aren’t caused by testosterone and won’t be fixed by neutering alone.

Why is my neutered dog still marking or aggressive?

If it’s been more than six weeks, the testosterone is gone, so the behavior is likely learned or stress-related rather than hormonal. Persistent marking or aggression is best addressed with training and a vet check to rule out medical causes.