The beneficial effect of insulin on the decreased gastrointestinal beta-adrenergic responses in experimentally diabetic rats has been reported. The effects of streptozotocin-induced diabetes and insulin on the isolated rat duodenum, precontracted with bethanecol, were examined in the absence and in the presence of protein synthesis inhibitors in isometric conditions. Streptozotocin-induced diabetes caused a decrease of the beta-adrenergic responses of the rat duodenum. In vitro insulin treatment corrected the decreased beta-adrenergic responses of rat duodenum due to streptozotocin-induced diabetes. In the presence of protein synthesis inhibitors, actinomycin D or cycloheximide, the beneficial effect of insulin on the decreased intestinal beta-adrenergic responses was significantly inhibited. These findings suggest that the beneficial effect of insulin in the rat duodenum might be related to a new receptor synthesis.