Insulin-like growth factor I (IGF-I) is shown to be involved in the regulation of pituitary hormones. High IGF-I concentrations were detected in hypothalamus and pituitary during adulthood. This study was undertaken to analyze the cellular distribution of IGF-I in the hypothalamo-hypophyseal system of the adult rat using immunocytochemical procedures. IGF-I was found to be widely distributed throughout the hypothalamus; it was present in the magnocellular neurons of the supraoptic, paraventricular, and accessory nuclei. Moreover, nerve fibres and puncta containing immunoreactive IGF-I were localized in the median eminence and the posterior lobe of the pituitary. These results support possible IGF-I neuromodulatory or neurohormonal action in the hypothalamus on pituitary hormone regulation.