The radioautographic distribution of mu, delta and kappa opioid binding sites was examined by in vitro radioautography in the rat hypothalamus using the highly selective ligands [125I]-FK 33-824, [125I]azidoDTLET and [125I]DPDYN, respectively. Levels of mu opioid binding sites varied considerably amongst hypothalamic nuclei. mu Opioid labeling was dense in the medial preoptic area, medial preoptic nucleus, suprachiasmatic nucleus and ventromedial nucleus, whilst the supraoptic nucleus, paraventricular nucleus, arcuate nucleus and dorsomedial nucleus were devoid of labeling. Delta opioid labeling was sparse throughout most of the hypothalamus; however, moderate binding densities were detected in the suprachiasmatic and ventromedial nucleus. kappa Opioid labeling was also scant throughout the hypothalamus with the exception of the suprachiasmatic nucleus which was very densely labeled. Our results indicate that the 3 opioid receptors types are differentially distributed within the hypothalamus, although a significant overlap exists. In general, the distribution of hypothalamic opioid receptors correlates well with that of opioid-containing terminal fibers and may represent the anatomical substrate for opioid involvement in the hypothalamic regulation of autonomic, behavioral and neuroendocrine functions.