Why is my neutered cat grumpy and hissing at other cats?

I would wait a few days before panicking about long-term effects. The stress of being hospitalized, the after-effects of anesthesia, and any medications can cause temporary behavior changes. If the cats weren't prescribed pain medications, then I would talk to your vet about that.

You should have him checked, or rechecked, by your veterinarian. Neutering typically does not result in more aggressive behavior. Additionally, any sudden behavioral change can be due to illness or injury. Once medical causes have been ruled out, you can focus on behavioral issues. Consider a pheromone product, such as Feliway Spray or plug in diffusers, to help decrease anxiety or aggression. Provide additional litterboxes, bowls, toys and bedding to prevent resource protection. You may have to separate them for a week or two then gradually reintroduce them while employing the measures described above.

It can take several weeks for testosterone levels to decrease after neutering. Until then, consider using a pheromone product, such as Feliway spray or plug in diffuser, to decrease any anxiety or aggression in the home. Provide extra litter boxes, bowls, toys and bedding to prevent food aggression or resource protection.

I am sorry to hear about your concern for Tumbles! I would suspect he smelled another male cat's urine/marking which got him excited/worked up. If it becomes a routine issue you can consider using Feliway multicat products https://www.feliway.com/us/Products/FELIWAY-MultiCat-Diffuser

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Sounds like Prince is still experiencing some signs associated with the male hormone testosterone. The testosterone levels drop after the neuter and should be decreasing now. Hopefully this behavior will ease up as the remaining testosterone gets out of his system. If things don't resolve or his signs worsen, I would advise that your veterinarian take a look at both of them to assess any problems. I hope this gets better soon.

It could be that she smells differently now that she had kittens, and Homer views her as a stranger. There is a method you can use to help them get along again. I would separate the cats, and slowly reintroduce them to each other. Take Homer aggressive, and put him in his own room with his own food, water, and litter box. Keep the door closed, and for a few days, feed them both treats on other side of door so they associate each other's smell with a good experience. Then let the other cat in Homer's room while he is in a cat carrier. They can smell each other, but not be in contact. If either becomes upset or show signs of aggression, remove the second cat from the room, and try again later. Praise them when they interact appropriately. When they are interacting well with each other through the carrier, you can let Homer out of his room, and see how he interacts with the other cat. Praise them when they interact appropriately, and separate them with any sign of aggression or upset. I would get a cat tree with lots of shelves to give the cats vertical space to get away from each other. I would keep Homer in his room when you go out until they are comfortable with each other.

Thank you for submitting your question regarding your cats. I suspect that the carrier ride and the surgery were stressful experiences for both cats. This can cause a cat to act out aggressively. You may want to keep the cats separate from each other for a few days to let them calm down. Then re-introduce them slowly and only when they can be fully supervised. Things should go back to normal between them, but it may take some time. I hope this information helps!