An oral liquid form of ivermectin was administered to 14 purebred Collies (12 rough coated, 2 smooth coated). All Collies were given ivermectin at dosages of 100 and then 200 micrograms/kg of body weight. Three of the dogs developed mild clinical signs of toxicosis (salivation, vomiting, confusion, ataxia, and tremors) with the 100 micrograms/kg dosage. After the 200 micrograms/kg dosage, 7 dogs (including 1 smooth-coated Collie) developed severe toxicosis (seizure-like activity, recumbency, nonresponsiveness, and coma). Because dogs that developed severe toxicosis were not retreated, only the 7 remaining dogs were given ivermectin at 600 micrograms/kg. Severe toxic signs were not observed in the dogs given the 600 micrograms/kg dosage, and only 1 of these 7 dogs developed severe toxicosis when given ivermectin at 2,500 micrograms/kg. Dogs that developed severe toxicosis were given supportive care while in the comatose state. All dogs recovered completely. The results indicated that Collies (including the smooth-coated Collies) have a wide range of sensitivity to ivermectin-induced toxicosis.