Background: The necessity of a 12-h fast before resting metabolic rate (RMR) is measured is often a barrier to measuring RMR. Objective: We compared RMR measurements obtained in the morning and afternoon and across repeated days to elucidate the magnitude and sources of variability. Design: Healthy men (n = 12) and women (H = 25) aged 21-67 y, with body mass indexes (in kg/m 2) ranging from 17 to 34 and body fat ranging from 6% to 54%, completed 4 RMR measurements. RMR measurements were made in the morning (after a 12-h fast and 12 h postexercise) and in the afternoon (after a 4-h fast and 12 h postexercise) on 2 separate days with the ventilated-hood technique. Body composition was assessed by dual-energy X-ray absorptiometry. Results: Mean (± SE) afternoon RMR was significantly higher than morning RMR on both visit 1 (1593.5 ± 35.6 compared with 1508.0 ± 31.5 kcal/d; P = 0.001) and visit 2 (1602 ± 29.3 compared with 1511.4 ± 35.9 kcal/d; P = 0.001). The 2 morning measurements (r = 0.93) and the 2 afternoon measurements (r = 0.93) were highly corretated, and no significant differences between measurements were observed. The mean difference between the morning and afternoon measurements was 99.0 ± 35.8 kcal/d (6%). Conclusions: Repeated morning and evening measurements of RMR were stable and highly correlated. Day-to-day measurements of RMR were not significantly different. RMR measured in the afternoon after a 4-h fast and exercise was ≈100 kcal/d higher than RMR measured in the morning.