The effect of muscular exertion of moderate intensity on blood sugar (BS), plasma levels of growth hormone (GH), glucagon, and cortisol (F) has been studied in endocrinologically normal children with short stature and compared with children with clinical diabetes mellitus and obese children with normal and diminished carbohydrate tolerance. In diabetic children, physical exertion induces a rise in plasma GH levels comparable to that in controls; in obese children with normal or with diminished glucose tolerance, the rise is considerably smaller. Physical exertion caused no change in F levels in the groups tested, although basal level in the obese children was significantly higher than in the controls. Basal glucagon levels were similar in all groups and showed no change on physical exertion. The behavior of GH and glucagon in diabetic children was comparable to that in the controls even where blood sugar level was high.