We had our 9 months old pup neutered last week and we've noticed he has become more aggressive towards our two cats. Is it natural or something went wrong? Any help will be appreciated.

I've heard stories like this before. Here are my thoughts about why this can be happening.

Neutered dogs are injured after surgery, and they have to be defensive to hide the fact they are not at full health. In the wild injury means weakness, and weakness makes an animal an easy prey for predators. Aggression becomes a defensive mechanism to make other animals back off so they don't guess about the weakness. It's a survival instinct.

You can speak with your vet about a pain medication, that might help reduce his aggression. Nevertheless, he will most likely continue being aggressive until full recovery. You may want to separate him from your cats for several days.

As to aggression in female dogs after spaying, many people believe there may be behavior changes because of hormones settling down but overall spaying doesn't result in aggression. If it happens, it's a sign of general behavior problems that have to be treated like in any other dog by training and establishing proper human leadership.

If I'm not mistaken it can happen with female dogs because of hormones. Spaying reduces the production of progesterone which is known to have calming effect so increased aggression is result of this. I don't know about male dogs though!
Andreas does this mean spaying female dogs doesn't reduce aggressive behavior? We have a female puppy 4 months old and we have to decide about spaying, if we'll do it and when. I hope she doesn't ever develop aggression problems but it's the first time I hear spaying cannot help.
 Raphael's reply was promoted to an answer.
We had our 8month old patterdale puppy spayed . She was calm and little shy at times to begin with BUT SINCE BEEN SPAYED IS GROWLING AND BARKING WE WISH WE NEVER HAD HER DONE. I would really think about is as now we having go vets regarding hormone replacement to see if helps. PRAYING IT DOES as it like our Ruby Having SOME X10 . She does not growl all times but her hormones are messed up. Will never get another female fixed again. Male yes as opposite effect he calmed down not as hyper HE WAS NEVER AGGRESSIVE TO BEGIN WITH EITHER.

Our 2 year old female has become dog reactive, alpha, and has heightened prey drive after being spayed. Her trainer said females have increased testosterone after a spay, and unlike males, will often become more aggressive than before.

Hi...I know this was posted a while ago but did your puppy mellow out again or was the new behavior permanent. We just spayed our 10 month old puppy, after her first heat, and she has been super irritated, anxious and energetic. She was the most chill, sweet puppy prior to be spayed. AND I"M FREAKING OUT!! really hoping it isn't a permanent change. any information is helpful!