We present a quantitative immunocytochemical study of GnRH migration by developmental stage. GnRH peptide was detected in cells of the olfactory epithelium at stage 19. Migration was initiated a few hours later at stage 20. Of interest is the observation that GnRH neurons paused at the central nervous system border for 3 days, entering the brain at stage 29. The major expansions of the GnRH population occurred at two points; stages 26 and 42. In one animal a third population expansion occurred after hatching, with the number of GnRH cells reaching 6600. To determine the site of origin of GnRH cells, embryos were exposed to tritiated thymidine and killed 5 h later. Most GnRH cells incorporated label in the olfactory epithelium; however, some autoradiographically labeled GnRH cells, possessing a neuronal morphology, were found in the olfactory nerve and the forebrain, suggesting that some GnRH neurons divide as they migrate. A cumulative labeling method employing tritiated thymidine was used to examine the birth date of GnRH neurons. Postmitotic GnRH cells were first detected at stages 19-21. At stage 24, a peak in GnRH neurogenesis preceded the increase in GnRH neurons expressing their peptide at stage 26. After stage 24, there was a gradual addition of postmitotic cells to the population through stage 35. A pulse-chase paradigm indicated that birth date did not influence the final GnRH cell distribution. Injections at stage 29, when 10% of the GnRH neurons are born, generated double labeled cells in all locations where placode-derived GnRH neurons reside.