My cat, Gracie, is a pretty hefty and muscular female, about three
years old. My new roommate's cat is considerably smaller and about the
same age. My cat, of course, is the aggressive one. The thing is, she
has been around other cats, introduced into their environments and had
one introduced into her environment without incident (other than the
typical 2-3 month hissing and swatting adjustment period). Most of the
time, all the cats involved were female, since I prefer females to
males.

The new situation is totally different. When I moved in with my new
roommate back around the middle of July, I introduced my cat by
keeping her movements restricted to my bedroom for the first couple of
weeks, to let the other cat get a whiff of the new arrival. However,
my cat didn't spend a lot of time near the door to that bedroom, since
she was pretty nervous about the new surroundings.

Eventually, the day came to let Gracie, my cat, have the run of the
house. That's when the trouble started: Gracie would attack my
roommate's cat, Samantha, whenever she saw her. Being much bigger and
stronger, she always gets the better of it. These are full-fledged cat
fights with extremely loud howling and fur flying (nobody has been
injured yet). Samantha has one area, way up high on a ledge where
Gracie cannot get to her, so she spends the majority of her time up
there. Of course, from the beginning, she has spit at Gracie whenever
she sees her approach (but I consider that pretty natural). Gracie
just waits for her to make a move and then the same thing happens.

How can a cat that showed no aggressive tendencies with other cats (at
least one of those cats was female and much smaller, just like
Samantha) how can she then turn into such an aggressor? More
importantly, how can we deal with this?

We've tried Feliway and several other pheromone formulas with no
effect. We've tried getting my shirt with my scent and the scent of my
roommate's cat combined and then putting that shirt where my newly
aggressive cat Gracie normally sleeps. And vice versa. No effect.
We've tried spraying my cat with water whenever we can catch her in
the midst of attacking: no effect. We've kept them in separate rooms,
but that's not a long-term solution.

Our next plan is to get them both in cat carriers and gradually move
the cat carriers closer together, but we have no idea if that will
help or hurt the situation.

Ideas urgently needed. Please reply in the group, as I don't check the
email address given (for obvious reasons).

Thanks in advance