Preferred ambient temperature (Ta) of male golden hamsters (Mesocricitus auratus) was measured repeatedly by placing the animals in a temperature gradient for 80 min. A total of 180 observations were made during the last 20 min of treatment in the gradient. The mean preferred Ta was 28.2 +/- 0.2 (S.E.) C. In another experiment the same animals were placed in a temperature-controlled chamber for 80 min while metabolic rate, evaporative water loss, thermal conductance, and colonic temperature were measured at Ta's of 14 to 34 C. The lower critical Ta, the Ta below which metabolic rate increased above the resting level, was 28 C. This Ta corresponds closely to the mean preferred Ta of the hamster when placed in the temperature gradient. Evaporative water loss was minimal at Ta's of 14 and 16 C and increased gradually with increasing Ta. Thermal conductance was relatively stable between Ta's of 14 to 28 C and then increased sharply with increasing Ta. The data from the hamster are qualitatively similar to the mouse in that the preferred Ta corresponds with the lower critical Ta. It appears that, for these rodents, the control of preferred Ta is critically related to the animal's metabolic requirements.