The ambient temperature (Ta) to house and study laboratory rodents is critical for nearly all biomedical studies. The ideal Ta for housing rodents and other animals should be based on their thermoregulatory requirements. However, fundamental information on the behavioral thermoregulatory responses of single- and group-housed rodents is meager. To address this issue, thermoregulatory behavior was assessed in individual and groups of CD-1 mice housed in a temperature gradient. Mice were housed in groups of five or individually while selected Ta and motor activity were monitored. Single- and group-housed mice displayed a circadian oscillation of selected Ta and motor activity with relatively warm T(a)s of approximately 29 degrees C selected during the light phase; during the dark phase selected Ta was reduced by 4 degrees C, whereas motor activity increased. Selected Ta of aged (11 months old) mice housed individually was approximately 1.0 degrees C warmer than the group-housed mice. Thermal preference of younger mice (2 months old) was similar for single- and group-housed animals. The operative Ta of mice housed in standard facilities was estimated by measuring the cooling rate of "phantom" mice modeled from aluminum cylinders. The results show that the typical housing conditions for single- and group-housed mice are cooler than their Ta for ideal thermal comfort.