Rats were anesthetized with pentobarbital, pentobarbital and atropine, inactin [5-ethyl-5-(1'-methyl-propyl)-2-thiobarbiturate], ether and inactin, or urethane. Cardiovascular and arterial acid-base parameters were monitored over a 3-hour period of anesthesia. Heart rate, arterial pressures, and pH progressively decreased with duration of pentobarbital anesthesia. Changes observed in rats anesthetized with the thiobarbiturate, inactin, were similar although generally less severe. Most subjects treated with the barbiturates were markedly hypercapnic. Urethane anesthesia was characterized by a higher and more stable heart rate and greater pulse pressure. Arterial carbon dioxide and bicarbonate levels in the urethane group were substantially lower at all sampling times than the values obtained in the barbiturate groups.