1. Subcutaneous or intrahypothalamic injections of capsaicin produce hypothermia in the neonate rat. Repeated injections with increasing doses of capsaicin result in unresponsiveness to this drug (capsaicin-desensitization). 2. Young rats aged 8-10 days which had received serial injections of capsaicin solution (a cumulative dose of 4.63 mg per animal) or of the solvent alone were subsequently tested as adults for their ability to thermoregulate. 3. On exposure to an ambient temperature of 41 degrees C, adult rats which had been capsaicin-desensitized as neonates were unable to thermoregulate against overheating by means of autonomic responses whereas control littermates could maintain normal rectal temperature. However, autonomic thermoregulation against cold was unimpaired in the desensitized rats. 4. Skin-cooling operant behavior in heat stress was impaired in adult rats which had been capsaicin-desensitized as neonates whereas their skin-heated behavior was not different from that of control littermates. 5. These results suggest that the central and peripheral warm-receptors responsible for thermoregulation in the neonate rat are functionally mature at least inasmuch as they form a part of thermoregulatory system involved in lowering body temperature and can be desensitized by capsaicin. Moreover, such receptors, once desensitized 8-10 days after birth, apparently do not regain their function and are not regenerated or replaced during subsequent maturation of the animal.