This is something we've done a few times over the years, it's never easy but usually gets better quickly once they've established who's boss. Usually it's the cats :lol: I've tried keeping them separate at first and have found this just increases the agitation and "strangeness" of it all. I make sure the cats have somewhere up high they can easily get to, and someplace they can go by choice that the pup can't get to - then let the pup to the floor and referee the ensuing chaos. Like I said ground rules are established pretty quick. 3 of our 4 inside cats get along fine with Donner, he chases the female sometimes (but only cuz' she runs) One other thing, we had to put away all Donners furry animal-like squeaky toys, increased his prey drive for the cats and he would try pouncing on them like a toy, not good for either one of them 8O Once they were put away though that behavior stopped. Good Luck!