The ability of rats to select a warm environment was studied as a function of postnatal age (birth to 13 days). Animals younger than 5 days demonstrated no choice response (movement to a warm compartment, 36-37 degrees C); however, they did demonstrate movement within the start compartment (23 degrees C). Increasing the motor capabilities of the pups, by injection of L-3,4 dihydroxyphenylalanine (L-dopa, 50 mg/kg), elicited a choice response in 4-5 day-old animals. Younger animals demonstrated no choice of a warm environment even though they moved considerably. Also there was no difference between L-dopa-treated and control animals in the magnitude of temperature change in pups isolated from their mother for 1 hr. The evidence suggests development of behavioral thermoregulatory mechanisms prior to abilities for internal regulation.