from what i know, dogs veiw cats as possesions in the house hierarchy and if they see them as a threat then they will chase them ("disipline" them to show who's boss) what you can do is show the dog alot of attention and give him treats when they cat is around and then stop when the cat leave, then he learns that cat here=good and cat leaves=not so good. but if he's just trying to play with the kitties then i'm not sure if that would be the best approach