We anesthetized 1-2 yr old harbor seals with pentothal (10 mg/kg, i.v.) and maintained anesthesia after tracheal intubation with 0.75 to 1% halothane in 20-50% 02 in N2. Ventilation was provided by either a Bird respirator or a Harvard respiratory apparatus, with a positive and expiratory pressure of about 5 cm H20. Core temperature (CT) dropped as much as 4°C within 1/2 hr of the induction of anesthesia, while skin temperature (ST) rose from near ambient (20­25°C) to within 1°C of the CT. CT then rose steadily for the duration of anesthesia, once to 39°C; ST was not measured after one hr. Systemic blood pressure (BP) dropped from 140/100 mm Hg before induction to 75/60 mm Hg within one hr of anesthesia and did not change thereafter. Cardiac output (Q) in the conscious seals averaged 11.88 l/min (n=68) while it averaged only 3.71 l/min (n=17) during anesthesia. This type of general anesthesia 1) appears to cause an immediate redistribution and (probably) radiation of body heat; 2) prevents adequate cutaneous heat loss in long term anesthesia; and 3) severely depresses the cardiovascular system. The BP and Q response may or may not pose a threat; the CT response requires monitoring and, occasionally, intervention.