LHRH release is induced by substance P (SP) in the rat hypothalamus. Recent immunocytochemical studies indicate that SP-immunoreactive axons synapse on LHRH neurons in the diencephalon of the rat, but this phenomenon has not yet been demonstrated in human. Therefore, in the present study we visualized the SP- and LHRH-immunoreactive (IR) elements in the human diencephalon and evaluated the close juxtapositions between them. The distribution of LHRH- and SP-IR sites were investigated in diencephalic sections of six, postmortem human brains by means of double-labeling immunocytochemistry. The LHRH-containing perikarya were located in the diagonal band of Broca, lamina terminalis cinerea, preopticoseptal, medial preoptic, and infundibular areas of the brain. The SP-IR fibers formed a network in the periventricular zone in the infundibular region, median eminence, and corpus striatum. The SP-IR cell bodies were located mainly in the infundibular region, median eminence, basal part of the periventricular area, dorsomedial subdivision of the ventromedial nucleus, and basal perifornical area of the tuberal region. The juxtapositions between LHRH-IR cell bodies and SP-IR varicosities were detected in the infundibular and periventricular regions. In these sites black, silver-intensified, SP-IR fiber varicosities abutted on brown, DAB-labeled, LHRH-IR cell bodies. Similar structures were detected between the SP-IR fibers and SP-IR perikarya. These findings suggest that the juxtapositions between the SP and LHRH systems may be the morphological basis of SP-controlled LHRH release in the human diencephalon. Moreover, the intimate contacts between SP-IR fiber varicosities and SP-IR cell bodies or axons indicate direct control of SP on the diencephalic SP release.