We evaluated the possible autocrine modulatory effect of insulin upon glucose metabolism and glucose-induced insulin secretion in islets isolated from normal hamsters. We measured ¹⁴CO₂ and ³H₂O production from D-[U-¹⁴C-] glucose and D-[5-³H]-glucose, respectively, in islets incubated with 0.6, 3.3, 8.3, and 16.7 mM glucose alone, or with 5 or 15 mU/ml insulin, anti-insulin guinea-pig serum (1:500), 25 µM nifedipine, or 150 nM wortmannin. Insulin release was measured(radioimmunoassay) in islets incubated with 3.3 or 16.7 mM glucose, with or without 75, 150, and 300 nM wortmannin. Insulin enhanced significantly ¹⁴CO₂ and ³H₂O production with 3.3 mM glucose, but not with 0.6, 8.3 or 16.7 mM glucose. Addition of anti-insulin serum to the medium with 8.3 and 16.7 mM glucose decreased significantly ¹⁴CO₂ and ³H₂O production. A similar decrease was obtained in islets incubated with 8.3 and 16.7 mM glucose and wortmannin or nifedipine. This latter effect was reverted adding 15 mU/ml insulin to the medium. Glucose metabolism was almost abolished when islets were incubated in a Ca²⁺-deprived medium, but this effect was not reverted by insulin. No changes were found in ¹⁴CO₂ and ³H₂O production by islets incubated with 3.3 mM glucose and anti-insulin serum, wortmannin, or nifedipine in the media. Addition of wortmannin decreased significantly insulin release induced by 16.7 mM glucose in a dose-dependent manner. Our results suggest that insulin exerts a physiological autocrine stimulatory effect upon glucose metabolism in intact islets, as well as on glucoseinduced insulin release. Such effect however, depends on the glucose concentration in the incubation medium.