A study was made of the effects of pretreatment with fractionated capsaicin on the changes in body temperature (Tb) induced in rats by exposure to cold and to warm environments, and by administration of histamine (40 mg X kg-1) or of pentobarbital (20 mg X kg-1). At the ambient temperature of 22 degrees C the Tb of capsaicin pretreated rats was lower than that of rats treated only with solvent (p less than 0.001). The capsaicin pretreated rats showed impairment of thermoregulation at both high and low environmental temperatures: their Tb rose to a higher level in a warm environment, while in a cold environment their Tb was lower than that of the control rats. Histamine and pentobarbital caused larger drops in the Tb of capsaicin pretreated rats than in the controls. These findings challenge the concept that capsaicin treatment acts exclusively on thermoregulatory mechanisms concerned with warmth sensation.