You should have gradually introduced them, but things happen, and looks
like the cats had other ideas.

 It's not going to happen overnight.  They need to get used to one another,
it could take a few weeks.  Hissing is normal, so is batting.  I have two 8
month old kittens that get hissy to any newcomers (I foster), after about
three weeks they're chasing each other through the house.  One of my older
ones, it took him months to accept the 8 month olds.

 In the meantime make sure you show them both affection.  Try to feed them
seperate  if you can until they get used to one another.  Good luck and have
patience.

 JoJo
 http://home.comcast.net/~zookeepr/fosters.html

 > Hi everyone,
 >
 > I have a one 1/2 year old black and white cat named Homer.  He's
absolutely
 > adorable, and he just came from a home with two other cats (one older
Calico
 > and one younger classic tabby).  So he's been around others, and he
wasn't
 > the last of the bunch to enter.
 >
 > Now he's alone, but I just last night introduced another cat to the
 > household... a 3 yr old small female classic tabby - very similar in
looks
 > to the youngest one at the other home.
 >
 > Needless to say, I didn't sleep too well last night, what with all the
 > hissing and batting going on, but they got introduced face to face much
 > earlier than I had wanted.  The new cat, well, she ran right out of my
room
 > and into the living room... into oncoming traffic (Homer) !!
 >
 > It's been almost 24 hours and while Homer has calmed down, they still
hiss
 > and swat at each other whenever they are near one another.
 >
 > Does anyone have any suggestions?  Should this sort of action fade in
the
 > coming days?  Is bringing a female into the home harder than another
male
 > for Homer?  We've only been in this home for nearly a month, so it's not
 > like he's been the "owner" for years!!
 >
 > Any advice would be greatly appreciated.  Feel free to leave a post or
reply
 > to roilsteve AT eastlink DOT ca
 >
 > Thanks!!
 > Steve

I did the samething (not knowing to introduce them slow) I had 2 cats
1 male 1 female (raised together since babies) and then added a 3rd
female cat. They hissed and batted for about 3 weeks - slowly
tappering off during that time.

I did get some feliway that someone here advised, and I do believe
that helped with them during the adjusting period. It cost a bit but I
really think it was worth it.

Now they all play together and you would never know that they were not
all raised together.

In fact I am redecorating again this spring ( now the bedroom gets new
furniture (just painted) and plan to get some feliway again. My male
cats stresses so bad when there is change or new people here so I hope
it helps him out.

Karryl