I know it depends on both the dog and the cat, but I need some general advice. I have had Cheshire for almost two weeks. She is 13 weeks old and weighs 3lbs. Obviously, she is too small to be left alone with Argon, so I put her in the bathroom when I leave. Argon has almost totally ignored her since she came home, though he did make an effort to play with her today. He pranced around and tried to swat her with a paw (Sadly, he does think that this is an appropriate invitation to play). She hissed and went under the bed, he got bored and wandered off. Argon has hunted outside, and killed both birds and baby rabbits, so he does have a prey drive. When I adopted him, Angry Puss was still living, and there was never an issue there, but Angry Puss was large and evil, and pretty much put Argon in his place from day one. How do you know when they can be left together? Chesh is kind of sad alone in the bathroom, but it is the only room with a door, and Argon can jump a baby gate.