hissing male cat

Question
I have two 3-year old cats, one female and one male, that have lived together since they were kittens.  They are spayed and neutered.  Recently, the male cat has begun to hiss and growl at my female cat whenever she comes near him.  After doing this, he either backs away and hides in another room or she runs away.  The female is now very leery about approaching him.  They are both indoor cats, but I have noticed that female cats are hanging around outside my house.  Could my male's reaction be because he's in heat and wants to get to the females outside or could it be something else?

Answer
Jean,

I think you have pinpointed the problem.  The cats hanging outside may be perceived as a threat by your neutered male.  He probably is exhibiting a fight/flight reaction.  When they do this, they no longer seem to recognize other cats in the household as friendly.  Usually time resolves the problem.  If he does not see the other cats, he should, eventually, be friendly with the female again.  I suspect she, too, is seeing these cats and is not very friendly towards the male, so the problem can feed on itself.

It is also possible he has a physical problem.  When cats are in pain, they can exhibit this kind of behavior.  A quick check with the vet should resolve whether this is an issue or not.

Best regards... Norm.