This paper presents the first detailed localization of luteinizing hormone (LH)-containing cells and fibers in the rat central nervous system. These immunoreactive elements were identified by four LH antisera, two directed against the intact LH molecule and two against LHb. Cell bodies, immunoreactive for LH were found throughout the rostral-caudal extent of the hypothalamic arcuate and ventromedial nuclei, the periarcuate area ventral to the ventromedial nucleus, and the retrochiasmatic area. Immunopositive fibers were traced to numerous structures within the brain including discrete regions of the hypothalamus, septal area, nucleus of the diagonal band, bed nucleus of stria terminalis, amygdala, thalamus, periaqueductal gray, raphe nuclei, brainstem reticular nuclei, locus ceruleus, parabrachial nucleus, dorsal motor nucleus of vagus, and the nucleus of the solitary tract, with a few fibers extending into spinal cord central gray. This pattern of fiber distribution corresponds closely with those described for fibers containing several other anterior pituitary hormones. The extensive projection for LH may provide neuroanatomical substrate mediating reproductive events as it does in the pituitary, or it may serve some modulatory function in brain which is independent of its role in reproduction.