Studies were undertaken to determine whether the insulin-like growth factors (IGF-I and -II), bind to specific membrane receptors in the pituitary and brain. Anterior pituitary glands, hypothalami, and brains (minus hypothalami) were obtained from adult male Sprague-Dawley rats (225-300 g) and 15,000 X g membranes prepared by differential centrifugation. Binding of 125I-IGF-I and 125I-IGF-II to all three membrane preparations was specific, time and temperature dependent, reversible, and increased in proportion to increasing concentrations of membrane protein or labeled ligand. Neither the pH of the assay buffer (6.5-8.5) nor the presence or absence of 1 mg/ml bacitracin had any significant effect on the levels of specific binding. In all three membrane preparations IGF-II specific binding was 3-5 times higher than that observed for IGF-I, and unlabeled IGF-II displaced either 125I-IGF-I or 125I-IGF-II better than comparable concentrations of IGF-I. All three membrane preparations showed similar low specific binding of 125I-insulin (1.3-2.2%) and negligible specific binding of 125I-rat GH (less than 0.5%). The presence of specific IGF and insulin receptors in rat anterior pituitary, hypothalamic, and brain tissue is additional evidence that IGFs and insulin are involved in modulating brain and pituitary function.