The growth hormone is a lipolytic hormone and somatomedin C mediates the metabolic effects of the growth hormone in many tissues. Growth hormone plasma levels are often depressed in obese individuals, and this low plasma level has been postulated as a reason for perpetuation of excess weight. Substantial weight loss in obese subjects improves growth hormone response to provocative stimuli. The present study investigated the effects of a 12-week progressive exercise program (consisting of jogging three times weekly) on growth hormone and somatomedin C plasma levels in five overweight males, 18-30 years of age and 15-40 percent above ideal body weight. Data collected at the beginning and end of the study included measurements for aerobic capacity, skinfold measures, body weight, fasting plasma growth hormone, glucose, and somatomedin C levels pretest and posttest. Mean body weight loss and decrease in skinfold measurement were significant during the program. Subjects' aerobic capacity increased. Changes in body weight accounted for 86 percent of the variability in somatomedin C changes; however, neither fasting plasma growth hormone levels nor growth hormone response to exercise stimulation changed significantly during the 12-week program. The responses suggest that somatomedin C is a more sensitive indicator than growth hormone of endocrine responses to exercise. Tables and figures with study results are appended. (Author/JD)