We analyzed temperature homeostasis in oxytocin-deficient (Oxt(-/-)) mice and found that Oxt(-/-) mice exhibited lower body temperatures than wild-type animals when they were exposed to cold. Oxt(-/-) mice also showed slightly more weight gain, but there were no obvious differences in the morphology of white and brown adipose tissues as between wild-type and Oxt(-/-) mice. In cold-exposed conditions, oxytocin neurons containing c-Fos immunoreactivity existed in the paraventricular nucleus of the hypothalamus. These results suggest that the central oxytocin neurons constitute part of the thermoregulatory system involved in maintaining body temperature in cold environments.