My vet suggested using Revolution for cats 0-5lbs for my rabbits after they picked up fleas while playing in their pen in the grass. I kept them seperated until it dried of course and the fleas were gone within hours and never had an issue with them after that. I would call your vet office and see if it would be safe for your guinea pig, since it was ok for my rabbits I don't see why it wouldn't be but I suppose you never know.