We examined the relationships among gender, sexual maturation, four-compartment model estimates of body composition, body fat distribution (magnetic resonance imaging for abdominal visceral fat and anthropometrics), aerobic fitness, basal and total energy expenditure, and overnight GH release in an ultrasensitive chemiluminescence assay in healthy prepubertal and pubertal boys (n = 18 and 11, respectively) and girls (n = 12 and 18, respectively). Blood samples were withdrawn every 10 min from 1800-0600 h to determine the area under the serum GH-time curve (AUC), sum of the GH peak heights (sigma GH peak heights), and the mean nadir GH concentration. GH release was greater in the pubertal than prepubertal subjects due to an increase in sigma GH peak heights (43.8 +/- 3.6 vs. 24.1 +/- 3.5 ng.mL-1, P = 0.0002) and mean nadir (1.7 +/- 0.2 vs. 0.7 +/- 0.2 ng.mL-1, P = 0.0002), but not peak number (4.3 +/- 0.2 vs. 4.5 +/- 0.2). The girls had a greater sigma GH peak heights (39.0 +/- 3.5 vs. 28.8 +/- 3.6 ng.mL-1, P = 0.05) and mean nadir concentration (1.4 +/- 0.2 vs. 0.9 +/- 0.2 ng.mL-1, P = 0.05) than the boys. Significant inverse relationships existed between sigma GH peak heights (r = -0.35, P = 0.06) or mean nadir (r = -0.39, P = 0.04) and four-compartment percent body fat for all boys but not for all girls or when combining all subjects. For all girls, significant inverse relationships existed between sigma GH peak heights (r = -0.39, P = 0.03) or mean nadir (r = -0.37, P = 0.04) and waist/hip ratio. Similar inverse relationships in all boys or all subjects were not significant. Forward stepwise regression analysis determined that bone age (i.e. maturation, primary factor) and gender were the significant predictors of AUC, sigma GH peak heights, and mean nadir. The influence of maturation reflects rising sex steroid concentrations, and the gender differences appear to be because of differences in estradiol concentrations rather than to body composition or body fat distribution.