The acute effects of bovine growth hormone (bGH) upon the release of glucagon and insulin from the isolated perfused rat pancreas were investigated in the presence of 5.6 mM glucose. Glucagon and insulin release occurred in response to 10(-9), 10(-8) and 10(-7)M bGH within 1-2 min and dissipated within 4 min; a significant increase was seen at 24 sec for glucagon and at 48 sec for insulin. In this concentration range, a dose-response relationship was established for both islet hormones. Upon the addition of 5 mM fumarate, glutamate, and pyruvate, glucagon and insulin release occurred also in response to 10(-11) and 10(-10)M bGH. In the absence of glucose, in response to 10(-7)M bGH, the magnitude of glucagon release was similar to that seen with 5.6 mM glucose; but the release of insulin failed to occur. In the presence of 16.7 mM glucose, the release of glucagon was blunted and that of insulin augmented. The data indicate that growth hormone can stimulate acutely the release of glucagon and insulin at physiological as well as pharmacological concentrations. These rapid and short-lived effects are likely to be direct and not mediated through the generation of somatomedin. The stimulatory effect of growth hormone appears to favor the release of glucagon under conditions of glucose lack, and of insulin under conditions of glucose abundance. These results indicate that growth hormone may have a tonic effect on islet hormone release.