In an attempt to understand the regulatory events that control the synthesis of gonadotropin-releasing hormone (GnRH) during the estrous cycle of the rat, we undertook a cellular analysis of time of translation of GnRH mRNA into protein. A specific antiserum, Rb 1076, which recognizes both the extended proGnRH as well as the processed form of the decapeptide was used for immunocytochemical staining. A cell was considered to be actively translating the pro-GnRH mRNA if elements of the rough endoplasmic reticulum (RER), including the outer nuclear envelope, were filled with reaction product. A synthetically quiescent cell contained only immunopositive neurosecretory granules. Cycling rats were killed at various times and all GnRH cells scored as being RER positive (+) or negative (–). On the morning of estrus almost all GnRH neurons in 5 out of 6 of the animals studied were synthesizing their unique peptide. The immunostaining in many of the cells at this time was very pale, suggesting a prior depletion. This was the only time point examined where near uniformity among individuals in a group was observed. At all other times considerable heterogeneity was observed among animals within a group. For example, at 17.30 h on the afternoon of proestrus 50% or more of the GnRH neurons were RER+ in half of the animals; the other half had values of 16–45% RER+. Synthetically active and inactive cells were found in close proximity in all animals. No regional differences were observed; all GnRH cell subpopulations from the level of the diagonal band of Broca through the hypothalamus reflected the population as a whole. These results suggest that only after the preovulatory surge of LH are all GnRH neurons synchronized to initiate synthesis of this neuropeptide.