Twenty dogs received a potentially lethal (15 mg/kg) dose of brodifacoum, a halogenated coumarin-type anticoagulant poison. Eleven were immediately treated with vitamin K1 daily for 5 days, either by intramuscular injections (2 mg/kg) or oral tablets (1 mg/kg). It was necessary to give further doses of vitamin K1 to most of the dogs for up to 2 weeks after the first treatment period to reduce their P times to normal levels (< 10 seconds). Four dogs were not given further vitamin K1 and two of these died of acute blood loss from an intrathoracic hemorrhage. Nine dogs received vitamin K1 (2 mg/kg by intramuscular injection) when clinical signs of anticoagulant poisoning were observed. Two dogs died suddenly without premonitory clinical signs of poisoning. The remaining 7 dogs showed various signs of anticoagulant poisoning 4 to 8 days after dosing and they received a 5 day course of vitamin K1. After this period one dog had a transient rise in its P time but this returned to normal without treatment, while another dog was treated on days 16, 20, 29 and 30. In conclusion, the authors recommended vitamin K1 therapy, 2 mg/kg by tablet or injection, daily for 3 weeks in cases of known or suspected brodifacoum poisoning in dogs.