Buffygirl,

How old is your cat? If he is a baby, you'll need to get him to the vet, as kittens can become dehydrated in a matter of 24 hours. Does the "throw-up" have whole pieces of food in it? If so, it could be that your cat is eating too much too fast. How is he acting? Does he go outdoors?? If so, he could've eaten some critter or something. Keep an eye on him. As long as he's eating, drinking and using the litterbox, he's probably just got an upset stomach.

Let us know how you make out.