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I have two cats one male thats 4 years old (hes nutered). I got a female kitten also nutered about 7/8 months ago. We have kept them in seperate rooms and tried the slow introduce. Positive reinforcement like treats when they see each other and playing with them on different ends of the room. Sometimes they will be okay for a little bit but then eventually the older cat will attack the younger one and he doesnt seem to stop even after all this time. We currently still have them separated and try here and there but the older cat just seems to not be getting better with her.