This study was performed to investigate the effect of weight-training (12 wk; 21 male subjects) on energy expenditure and substrate utilization during sleep. Sleeping metabolic rate (SMR) as measured in a respiration chamber was calculated according to three procedures: the lowest mean energy expenditure of a shiftable 3-h interval between 0:00 and 6:00 (SMR3) and SMR over predetermined intervals from 3:00 to 6:00 (SMR3-6) and from 0:00 to 6:00 (SMR0-6). In analogy with SMR the corresponding respiratory quotients were expressed as RQ3, RQ3-6, and RQ0-6. Changes in body composition were assessed from changes in body weight, body volume (densitometry), and total body water (deuterium dilution). Weight-training induced an increase in fat-free mass (+1.1 +/- 1.3 kg; P < 0.001) and a decrease in fat mass (-2.3 +/- 1.5 kg; P < 0.001) and body weight (-1.1 +/- 2.1 kg; P < 0.05). There was no significant change in SMR, irrespective of the way SMR was expressed. Only RQ3 decreased significantly (from 0.82 +/- 0.04 to 0.79 +/- 0.02; P < 0.05). Remarkably RQ3, RQ3-6, and RQ0-6 were highly negative correlated with the pre-training RQ (r = -0.93, -0.91, and -0.90, respectively: P < 0.001) resulting in a diminished variation in post-training RQ (P < 0.001). These results suggest that weight-training has no effect on SMR but increases relative fat utilization in low fat oxidizers and vice versa for individuals displaying high pre-training lipid oxidation.