Cardiovascular and pulmonary effects of propofol, a relatively new nonbarbiturate intravenous anaesthetic, were assessed and compared in 22 male and female dogs. Dogs in group 1 did not receive any premedication prior to 6.6 mg/kg IV propofol, group 2 was premedicated with atropine (0.02 mg/kg IM) and the alpha 2-agonist medetomidine (10 micrograms/kg IM), and group 3 received the same premedication agents as group 2, but the medetomidine effects were reversed by the alpha 2-antagonist atipamezole (30 micrograms/kg IV) after 30 min of anaesthesia. Each dog in groups 2 and 3 received a propofol induction dose of 2.2 mg/kg IV. The anaesthetic duration was shortest with propofol alone and prolonged with medetomidine as a premedication which was reversible with atipamezole. In group 1, the most prominent effects were a temporary drop in diastolic arterial blood pressure (26% and 24%) at 2 and 5 min post-propofol, respectively and a drop in respiratory frequency (41%) 2 min after propofol induction. Similar respiratory depression was observed in groups 2 and 3 (20% and 48%, respectively) at the same time. Apnea was not observed. An increase in systemic arterial blood pressure was observed throughout the trial in groups 2 and 3 until dogs recovered or were reversed with atipamezole. Medetomidine significantly reduces propofol dosage requirements. Safe and effective injectable anaesthesia was produced by propofol in this group of dogs. The frequency of respiratory depression would suggest in clinical usage, the practitioner should be aware oxygen supplementation is the treatment of choice should apnea occur.