The sites of luteinizing hormone-releasing hormone (LH-RH) neurons and their axonal pathways in the hypothalami of rats and mice were studied by the immuno-globulin-peroxidase bridge technique and by the immunofluorescent isothiocyanate technique, using rabbit antiserum to synthetic LH-RH. Both of these techniques give similar results concerning LH-RH localization both in rats and mice. The immunoreactive LH-RH positive neurons are about 18–26 μ in diameter, mostly angular, pyriform or spindle shaped and very rarely oval or round. Most of these neurons are generally scattered in the hypothalamus. In the preoptic and medial prechiasmatic areas, ventromedial, arcuate nuclei and ventro-lateral-premammillary body, the neurons are numerous and mostly in groups of 3–6. In other areas of the hypothalamus surrounding the suprachiasmatic area, supraoptic area, paraventricular nuclei and the anterior hypothalamic area, the LH-RH neurons are few and scattered. Very few neurons occur in the median eminence or in the infundibular
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