The experiment was designed to study the effects of sodium pentobarbital on behavioral thermoregulation in rats and mice. Male Sprague-Dawley rats were given intraperitoneal (i.p.) injections of sodium pentobarbital in doses of 0, 1, 5, 10 or 15 mg/kg and male CBA/J mice were given doses of 0, 5, 10, 15 or 30 mg/kg. The animals were immediately placed in a temperature gradient which allowed them to select their preferred ambient temperature (Ta). The preferred Ta of rats increased following a 10 mg/kg injection of sodium pentobarbital. However, the barbiturate had no effect on the preferred Ta of mice. The data suggest a peripheral action of sodium pentobarbital in rats which results in hypothermia and an appropriate compensatory behavioral selection of a warmer Ta.