Objective: The present study aimed at evaluating the acute effect of increasing doses of octreotide (OCT), a long-acting somatostatin analogue, on glucose tolerance and insulin secretion. Methods: A standard and two other oral glucose tolerance tests 30 min after subcutaneous administration of OCT were performed in randomized order in each subject. Obese subjects received 10, 25, or 50 µg of OCT; control subjects received only 10 and 25 µg. Fifteen obese and 10 control subjects were studied; all of them had a normal glucose tolerance. Plasma glucose and insulin levels were measured at times –30, 0, 30, 60, 90, 120, 150, and 180 min after the glucose tolerance test. Results: The results demonstrated that, following OCT administration, both control and obese subjects developed a reduced glucose tolerance, a delayed glycemic peak, and an increase of late plasma glucose values. Fasting as well as stimulated insulin secretions were higher in obese subjects as compared with controls, and insulin secretion was inhibited in a dose-dependent manner by OCT. Conclusions: These data indicate that the action of OCT might be due to at least two different cooperative mechanisms: (1) a delayed glucose absorption, as suggested by the delay of glycemic peak, and (2) a direct or vagal-mediated effect on β-cells, as suggested by the reduction of the area under the curve values in spite of the elevated late glycemic levels. It is noteworthy that doses of OCT as low as 10 and 25 µg are sufficient to inhibit insulin secretion both in normal and obese subjects.