We examined changes in serum leptin concentrations and thermogenesis in 12 healthy men (39 +/- 2 years, BMI 22.8 +/- 0.3 kg m-2) during a 4 h euglycaemic hyperinsulinaemia performed on a control day and after a day of competitive marathon runs. As compared with the control day, after the marathon, baseline FFA concentration (543 +/- 0.73 vs. 955 +/- 96 mumol L-1, P < 0.05), lipid oxidation rate (0.8 +/- 0.1 vs. 1.2 +/- 0.1 mg kg-1 min-1, P < 0.01) and energy expenditure (5.2 +/- 0.1 vs. 5.5 +/- 0.1 kJ min-1, P < 0.01) were elevated. Baseline serum leptin concentrations were similar on the control and postexercise days and increased during insulin infusion by 35-45% on both days (P < or = 0.01). Baseline serum leptin concentration correlated directly with serum insulin (r = 0.65, P < 0.05) and cortisol (r = 0.64, P < 0.05), and inversely with serum growth hormone concentration (r = -0.66, P < 0.05). In the postexercise study, the rise in energy expenditure during insulin clamp correlated with serum leptin concentration as determined before (r = 0.61, P < 0.05) or at the end of insulin infusion (r = 0.55, P < 0.05). Thus, in healthy individuals with normal body weight: (1) hyperinsulinaemia increases serum leptin concentrations; (2) a rise in energy expenditure correlates with serum leptin concentration. These data suggest that leptin is involved in the regulation of energy expenditure in humans.