Check out FELIWAY for Cats - Official Site I have seen the plug in version of this product used many many times with great success. It relaxes the cats a little bit, making them feel more at ease. Nothing like Prozac or a heavy duty psychotropic med, but a pheromone that takes the edge off a little bit. I would also go back to the beginning with the cats, separating them and slowly introducing them over several weeks. First, give each set of cats items with the other cats scents on them, then place them in an area where they can see each other but not directly interact, then start short visitations. Make sure you have a litter box for each cat (ideally, one more box than cats) as well as food and water bowls for each kitty. They may never all be buddies, but they could be roommates and learn to live with one another.