In the present investigation we determined metabolic rate during sleep (SMR) as a function of physical activity (PA) in normal daily life. Subjects were nine females and six males, with an average age of 30 yr (range 19-48) and an average Quetelet Index of 32 (range 14-62). The level of SMR was expressed as an index; SMI = SMR/BMR, where SMR was measured in a respiration chamber while subjects were asleep and basal metabolic rate (BMR) was calculated with the equation from Harris and Benedict. Physical activity, calculated from average daily metabolic rate (ADMR) and BMR, was expressed as an index: PAI = ADMR/BMR, where ADMR was measured with doubly labeled water and BMR was calculated as indicated above. Individual values of SMI were 0.8-1.2. SMI was positively correlated with PAI (r = 0.85, P less than 0.001). Subjects with either a high or a low Quetelet Index were concentrated in the lower range of PAI compared with normal-weight subjects. In conclusion, subjects with an extremely low or high body weight generally show a lower than expected SMR associated with a low physical activity.