Revolution, in the right amount, provides protection against heartworms. I have one study (with separate results for many different pets, including ferrets) that suggests that 18 mg. of selemectin per month for a 1-kg ferret should provide protection against heartworms. Of course, this assumes that the ferret is not already infected. Revolution for cats sized 5-15 lbs. has 45 mg of selemectin per 0.75 ml vial. I divide one vial equally between two ferrets once a month, so each gets 22-23 mg. of selemectin per month. I do this from April -- October, the time of year that mosquitos are around here in CT. If anyone wants a copy of that study, which is in PDF format, email me privately. Jeff In Loving Memory of Neo, Trinity, Morphy, Possum, Pip, Pop, Sabrina, Minnie Mouse, Hunny, Misty, Frodo, Baggins, and Mr. Parker Caring for Luna, Baby Girl, Dozer, Boomer, and Zoomer [Posted in FML 7119]