Anesthesia was induced in the harbor seal (Phoca vitulina) with an intravenous injection of 10 mg/kg thiopental sodium; this was followed by halothane (1%) anesthesia for up to 9.5 h. Cardiac output was reduced to 30% of the pre-anesthesia value (from an average of 11.5 1/min to 3.5 1/min) while systemic blood pressure fell from an average of 150/110 to 80/60. Arterial oxygen partial pressures were somewhat depressed (58-72 Torr) during ventilation with air. Heart rate became stable at 90-100 beats/min. Hypothermia was an occasional problem during the first hour of anesthesia, but this trend reversed and gave way to hyperthermia during prolonged anesthesia.