In rats chronically implanted with an electromagnetic flow probe around the ascending aorta, terminal aorta, or superior mesenteric artery as well as arterial and venous indwelling catheters, changes in cardiac output, hindquarter flow, splanchnic flow, and arterial pressure on pentobarbital anesthesia were observed. On intravenous injection of pentobarbital sodium at 30mg/kg, arterial pressure dropped acutely from an average value of about 105mmHg to a minimum of about 75mmHg in about 5min and then gradually recovered to an average level of about 90mmHg in 30min. Cardiac index gradually decreased about 30% on the average in 30min. Hindquarter flow decreased about 25%. Superior mesenteric flow first increased about 40% within 5min and then returned almost to the premedication level in 30min. In adrenalectomized rats there was no decrease of hindquarter flow on pentobarbital anesthesia. The increase in superior mesenteric flow immediately after pentobarbital injection remained almost unchanged after adrenalectomy or splanchnicectomy. It is concluded that an appreciable portion of the decrease in cardiac output on pentobarbital anesthesia is induced by inhibition of tonic adrenomedullary secretion which has a dilating effect on muscle blood vessels through stimulation of β-receptors. The marked increase in splanchnic flow immediately after pentobarbital injection, which is responsible for the concomitant drop in arterial pressure, is considered to be induced by a direct inhibitory effect of the anesthetic on splanchnic blood vessels.