Mice exposed to subanesthetic partial pressures of N2O (0.25 to 0.75 atm) or N2 (5.7 or 11.33 atm) and allowed to choose between a warm and a cool environment showed a marked preference for the cooler environment. This behavior was associated with the onset of hypothermia, with deep body temperature falling by up to about 3 degrees C, usually to a new, steady level. Both the length of time spent in the cooler environment and the degree of the hypothermia produced increased with the partial pressure of N2O or N2 used. The effects of N2O on behavioral thermoregulation and body temperature were reversible. There was a correlation between anesthetic potency and the ability of both gases to alter thermoregulation, suggesting that the effect of these agents on thermoregulation was caused by the same molecular interactions as those which underlie anesthesia. Since both gases elicited changes in behavioral thermoregulation promoting rather than opposing the onset of hypothermia, it is concluded that they may have acted to lower the level at which deep body temperature was being regulated.