Intracerebrally-administered neurotensin produces a marked hypothermia in a variety of mammals. In this study, prior adaptation to a cold environment was found to significantly antagonize the hypothermia produced by intracisternally-administered neurotensin in mice. This antagonism required both previous exposure to cold ambient temperatures and cold exposure immediately prior to, or simultaneously with, neurotensin administration. The antagonism of neurotensin-induced hypothermia by prior cold-adaptation was blocked by indomethacin, but not by acetylsalicylic acid, suggesting that brain prostaglandin synthesis may be essential for this newly-discovered phenomenon.