Tolbutamide-glucose tolerance test was carried out on nine non-diabetics and eight diabetic patients in order to evaluate the endocrine function of the pancreas. In non-diabetics, blood sugar decreased thirty minutes after tolbutamide injection and rose to 135mg/100ml in the mean value after glucose loading. In diabetics a decrease in blood sugar after tolbutamide injection was small and blood sugar reached 229.8mg/100ml after glucose loading. Plasma insulin levels in non-diabetics, fasting and 30 minutes after tolbutamide injection, were not different from those of diabetics, but they rose to 81.5 μU/ml after glucose loading, while plasma insulin in diabetics reached 30.8 μU/ml after glucose loading. The difference in plasma insulin after tolbutamide-glucose loading between non-diabetics and diabetic patients is significant. There is a significant correlation between the maximal blood sugar and maximal plasma insulin after tolbutamide-glucose loading (r=-0.69). The tolbutamide-glucose loading might be of value for the evaluation of the pancreatic islet function.