Proper adjustment of energy metabolism is one of the most important strategies in small mammals in response to variations in ambient temperature. The present study was designed to test the roles and mechanisms of metabolic thermogenesis in the adaptive regulation of body mass and body fat under warm temperatures. Body mass, energy intake, metabolic thermogenesis were determined in striped hamsters that were acclimated to warm temperature (30°C) for 1, 3 and 4 months. Cytochrome c oxydase (COX) activity and mRNA expression of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) were also measured. The results showed that warm temperature had no effect on body mass, but significantly increased body fat. Digestibility increased significantly in warm-acclimated hamsters compared to those maintained at 21°C, but the gross and digestive energy intake were significantly reduced. Nonshivering thermogenesis (NST) was reduced significantly under warm temperature. COX activity in brain, liver and heart was significantly reduced, and BAT COX activity and UCP1 mRNA expression significantly were decreased in warm-exposed hamsters compared to those maintained at 21°C. These findings suggest that the decrease in the energy expended for metabolism and thermogenesis compensates for the decreased energy intake, resulting in a positive energy balance, and consequently contributes to the increase in fat deposition. Down-regulation of metabolic rate of brain, liver, heart and BAT is the physiological mechanism underpinning the reduction of metabolic thermogenesis, which may be involved in the fat deposition in response to warm temperature.