We studied insulin receptor binding and carbohydrate metabolism in 10 patients with severe hyperprolactinaemia and compared the findings with those obtained in 20 healthy control subjects. Insulin binding to monocytes and erythrocytes was significantly decreased in the patients with an excess of prolactin. Scatchard analysis of binding data indicated that a decrease in the number of receptors rather than in receptor affinity seems to be the prevailing cause of lowered binding in hyperprolactinaemic patients. Furthermore, patients with severe hyperprolactinaemia demonstrated significantly elevated blood glucose levels following oral or intravenous glucose load despite having significantly increased insulin levels after glucose administration. The infusion of insulin induced a delayed hypoglycaemic effect and a decreased inhibition of endogenous insulin secretion, as indicated by the suppression of C-peptide in the hyperprolactinaemic patients. The present data indicate that severe hyperprolactinaemia is associated with an insulin-resistant state, which seems to be caused, at least in part, by a down-regulation of insulin receptors.