Preferred ambient temperature (Ta) was measured in nine mice of the BALB/c strain using a temperature gradient. When tested over four consecutive days the mean preferred Ta was 30.9C. In another study using the same animals, metabolic rate, evaporative water loss, whole-body thermal conductance, and colonic temperature were measured at specific Ta's over a range of 18 to 34C. The preferred Ta of 30.9C was associated with (1) the lower critical Ta (i.e., Ta below which metabolic rate increased) (2) a 57% elevation in evaporative water loss when expressed in units of mg water evaporated per ml consumed oxygen, (3) a thermal conductance that was 92% above baseline levels, and (4) a normothermic colonic temperature of between 37.0 to 37.5C. The data indicate that mice will select an environmental temperature associated with a minimal energy expenditure but a higher than minimal rate of evaporation and higher thermal conductance.