The components of energy expenditure, total metabolic rate (TMR), resting metabolic rate (RMR), thermogenic response to feeding (TEF), activity, and cost of activity were measured in fed and fasted mice housed at 22 and 30 degrees C. Mice housed at 22 degrees C had more than two times larger TMR and RMR. Mice at 22 degrees C were less active when fasted but more active when fed. Cost of activity was nearly doubled in the fasted and in the fed state. Analysis of the short-term relation between TMR, RMR, and bouts of activity showed that, at 22 degrees C, the bouts of activity induced a decrease in the intensity of RMR that reflected the reduced need for thermal regulation induced by the heat released from muscular contraction. This phenomenon induced a considerable underestimation of TEF and prevented its reliable measurement when mice were housed at 22 degrees C. Correlation between TMR and activity measured across time in individual mice was very strong at both 22 and 30 degrees C, but the correlation measured across mice was much weaker at 30 degrees C and no longer significant at 22 degrees C. We suspect that this phenomenon was due to the fact that RMR is a much more reliable predictor of TMR than activity. RMR is more variable at 22 degrees C than at 30 degrees C because of heat transfers between thermal regulation and heat released by other discontinuous processes, such as activity and TEE Therefore, more noise is introduced into the correlations performed across multiple mice between TMR and activity at 22 degrees C. On the other hand, it should be kept in mind that the doubling of TMR and RMR at 22 degrees C is fueled by an increased non-shivering thermogenesis that can obviously modify how the mouse responds to pharmacological and nutritional challenges. Taken together, these results suggest that in pre-clinical studies, mice should be housed in conditions where thermal regulation is limited as is generally the case in humans. However, the increased sensitivity of mice to small changes in ambient temperature can also be used as a versatile tool to investigate the role of thermal regulation on the energy balance equation in humans.