In order to identify metabolic cues involved in the control of onset of male human puberty, 70 pre- and peri-pubertal boys, aged 9.21 +/- 0.03 years at the start of the study, had their lean body mass (LBM), basal metabolic rate (BMR) and total daily energy expenditure (TDEE) measured at 6-monthly intervals on three occasions each. Hormonal pubertal activity was assessed by measuring early morning salivary testosterone (ST) at each assessment. A subgroup of 44 boys, who first showed an increase in ST at their third assessment, was identified. In the general population at this stage of development, and in the overall 70 boys studied here, BMR/LBM decreases significantly with age (from 0.185 +/- 0.001 to 0.180 +/- 0.001 MJ/day per kg over 6 months, p < 0.01, ANOVA), since both are increasing, but LBM at a greater rate than BMR. The 'imminently pubertal' group of 44 boys showed no decrease in the ratio BMR/LBM (0.182 +/- 0.001 MJ/day per kg) during the 6 months between their first two assessments despite the normal increase in LBM, indicating that BMR rises at a similar rate to LBM during this specific period of development. This observation supports the hypothesis that energy balance plays an important role during the events leading up to the onset of puberty and subsequent fertility.