I am 3 weeks into introducing new cat (2 year old male) to old cat (8 year old female). I've read the articles on this site and have swapped bedding, litter trays, left the other one's fur in other's room. What I am doing now is putting one of them in carrier in presence of the other. My old cat will growl and hiss sporadically and new cat typically does not react aggressively to her - he will lie down in front of her, seemingly very submissive. I have now been caught out 3 times when I have thought they are able to tolerate each other. As soon as they are face to face and my old cat starts moving, new cat chases her and they end up in a fight - usually under a table so it is difficult to see what is going on, There is a lot of noise and they seem to be facing each other and clawing each other. No injuries as yet but I am worried. Why does new cat act in non-aggressive manner but then chase her. Old cat hisses and growls but does not act violently towards him until he starts to chase. I am very worried that he will hurt her. Should I keep going with keeping one in carrier? How long for each time and how often? Right now I am doing this once or twice a day for 15 minutes. My partner is already starting to say we may have to return new cat if he cannot get on with other cat. I do not want to do this. New cat had been at vet hospital for 4 months recovering from broken pelvis from road accident and I believe he deserves a bit of stability now.