Insulin secretion and glucose tolerance were examined in 6 highly conditioned athletes in comparison with a control group of 115 normal healthy persons. During glucose infusion the athletes showed low insulin secretion although there was no difference in the levels of blood glucose compared to the control group. It is concluded that under physiologic conditions the extent of insulin secretion is not dependent only upon the blood glucose levels. The results show that a lack of insulin response can occur as a consequence of adaption to physical training. A reduced insulin response, therefore, does not necessarily indicate a diabetic or prediabetic state.