Summary  Serum growth hormone, immunoreactive insulin, plasma fatty free acids and blood sugar were measured during oral glucose, cortisone primed oral glucose and intravenous glucose tolerance tets and during intravenous tolbutamide test in 25 normal and 24 potential diabetic (offspring of two diabetic parents) males, closely matched for weight and age. Only potential diabetics with normal blood sugar levels during the oral, cortisone-primed and intravenous glucose tolerance tests were selected for study.-Mean serum, growth hormone concentrations were significantly higher in the potential diabetic group at one or more intervals in each of the tests. The potential diabetic group showed a paradoxical rise in growth hormone during the first 60 min of the oral glucose tolerance test and to a less marked degree in the cortisone-primed oral glucose tolerance test.-Serum insulin was significantly reduced in these potential diabetics (who had been selected for their normal carbohydrate tolerance) during the oral, cortisone-primed and intravenous glucose tolerance test but not during the tolbutamide test. The presence of normal blood sugar responses and reduced insulin levels suggested increased sensitivity to endogenous insulin in the potential diabetic group, despite elevations in serum growth hormone.-Abnormal growth hormone responses occurred in 50% of the potential-diabetic group and an abnormal response in one test was usually associated with abnormal responses in each of the other types of tests.-When the potentialdiabetics were subdivided into those with (responders) and those without (non-responders) an abnormal growth hormone response, it was the responders who as a group showed increased sensitivity to endogenous insulin. Thus the abnormal growth hormone responses observed appeared to be associated with acute insulin-like effects, rather than the more usual diabetogenic action of growth hormone.