Postabsorptive resting metabolic rate (RMR) and diet-induced thermogenesis (DIT) were repeatedly assessed on different days in 10 young men of normal weight on mornings and afternoons. No significant diurnal variation was found in RMR, DIT, total postprandial substrate oxidation, and overall postprandial nutrient balances. The pattern of postprandial substrate oxidation indicated an increased glucose oxidation in the first hour after ingestion of a meal on morning tests compared with afternoon tests. This was probably related to differences in the degree of the postabsorptive state, ie, 12-14 h on morning tests and 6-7 h on afternoon tests.