Cinda - This is what Dr. Klein gave me for heartworm preventive: 0.3cc injectable ivermectin in one ounce propylene glycol (100mcg/cc) Dosage: 0.1cc/lb body weight once monthly; i.e. 0.3cc for a 3-lb ferret. Your vet can call to confirm with Dr. Klein at 915-855-4100. Since 2 - 3 adult worms can kill a ferret, I consider this a very worthwhile investment. Hope this helps. Blessings, Carla Smith Rio Grande Domestic Ferret Club [Posted in FML issue 1554]