Endogenous insulin secretion after different stimuli was determined in insulin requiring diabetics without circulating insulin antibodies. Four groups of non-obese diabetics were investigated and compared with 111 controls. Group I: 14 patients with mild diabetes, not yet requiring insulin; diagnosis before the age of 30 years. Group II: 19 ketonuric patients just before being started on insulin treatment. Group III: 18 patients during remission after an average of 16.5 months' insulin treatment. Group IV: 13 patients with no remission period or relapse after an average of 19.5 months on insulin treatment. Blood glucose and immunoreactive insulin were measured during fasting and after iv secretin, iv tolbutamide, iv GTT, and oral GTT, followed by combined iv tolbutamide and glucagon stimulation. A considerable insulin secretion could be demonstrated in group I, whereas in group II only a very low insulin peak was obtained after secretin and the combined injection of glucagon and tolbutamide. In group III considerable insulin secretion was demonstrated, whereas in group IV only a very low insulin peak was obtained. A significant correlation between the degree of metabolic control and endogenous insulin secretion was found.