The distribution of GnRH immunopositive cells in the rat was studied from day 13 of fetal life to day 10 of postnatal life with the aid of a GnRH antibody (H-16) generated against the decapeptide p-Glu1-LHRH in the rabbit and following a PAP immunohistochemical staining protocol on 70 um serial Vibratome sections. The first appearance of GnRH cells was observed simultaneously in both intra- and extra-cerebral regions on day 16 of fetal life. At all stages of development, at least 10 times more GnRH cells were found in the brain than in extracerebral areas. The population of GnRH cells was maximum in the nasal mucosa on day 20, and in intracranial, extracerebral regions (along the anterior cerebral artery and the terminal nerve) on day 22 of fetal life: it declined thereafter. The population of GnRH cells in the brain continued to increase until the day of birth and remained unchanged thereafter. Our findings are in accordance with the theory of nasal placodal origin of the GnRH neuronal system, but also suggest that in the rat, shifting of the cells from the nasal placode to the brain is somewhat preceding the onset of GnRH synthesis. Our other field of endeavor was to examine the GnRH axon terminal fields in the course of development. On day 18 of fetal life, the vascular organ of the lamina terminalis, the median eminence and the medial habenular nuclei received their GnRH axon terminals. On day 19 the subfornical organ, mammillary nuclei and the central nucleus of the amygdala, and on day 21 the periaqueductal gray and the interpeduncular nucleus became invaded by GnRH axons. These data serve the better understanding of the onset of the different functional activities of the GnRH system in the rat.