Once you've been to the vet and positiviely enforced the "doesn't appear to
be sick" statement, check around the outside of your house for strays.  Even
if the cat can't see them, he might smell them - especially if they're
spraying/marking their territory.

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Did you do something like bring one to the groomers?
Cats identify others by smell, changing that smell
will mean one cat will see the other cat as a "new" cat.

Are there any signs that she may be unwell? If she's unwell, he'll smell it
and that could well provoke the behaviour (happens with my cats).

Did the cat change her brand of perfume recently? Toms don't like that.

Carola

When was the last time both kitties were to the vet? They both may need to
go in and get checked out. These babies now enter the senior course of
vetting and that's twice a year not just once. Some people would disagree on
this but as a cat ages many things change in their body.

Their nutritional needs change.
Tarter build up and gum disease.
Diabetes.

The list goes on. It's better to catch these things before they start
showing signs. Cats can be deathly ill and not show it until they are almost
dead.

Shadow Walker