1. Decreased beta-adrenergic responses have been reported in both gastro-intestinal tract and atrium of experimentally-induced diabetic rats. The present study was undertaken to investigate in vitro effects of insulin on decreased beta-adrenergic responses of the duodenum and atrium from streptozotocin-diabetic rats. 2. Insulin incubation (16.67 micrograms/ml) in bathing medium for 5 hr enhanced the decreased beta-adrenergic responses in the diabetic rat duodenum, but not those in the diabetic atrium. Incubation of bovine insulin with anti-bovine insulin antibody in the test-tube inhibited the improving effect of insulin on the decreased beta-adrenergic responses of diabetic rat duodenum. 3. In vitro treatment with the same dose of bovine insulin in bathing medium caused a decrease in the beta-adrenergic responses of the atria from both non-diabetic and diabetic rats. Anti-bovine insulin antibody also abolished the inhibitory effect of insulin on the rat atria. 4. These results strongly suggest that the experimental diabetes affects beta-adrenergic responsiveness of the rat gastro-intestinal tract through a different mechanism from that of the rat myocardium.