The luteinizing hormone-releasing hormone (LH-RH) content of 16 separate hypothalamic nuclei and of the median eminence have been determined by radioimmunoassay in the rat. High concentrations of LH-RH were measured in the median eminence and in the arcuate nucleus. Other nuclei of the hypothalamus, as well as the medial preoptic nucleus, contained little or no LH-RH. The amount of LH-RH stored in the median eminence is about twice that found in the arcuate nucleus, but the concentration in the median eminence is 7.7 times higher than in the arcuate nucleus. Within the arcuate nucleus 73% of the LH-RH is present in the middle part (NA III) of the nucleus. (Endocrinology 95: 554, 1974)