The quantitative distribution of LHRH neurons within the central nervous system of male and female rats during postnatal development was investigated with light microscopic immunocytochemistry. Approximately 1,300 immunoreactive LHRH cells were found within the forebrain at all ages. The topographic distribution of LHRH neurons was adultlike in the earliest neonatal stage examined (2 days postnatal) and remained similar throughout development. Two LHRH cell subtypes, smooth LHRH cells and LHRH cells with spinelike processes (irregular LHRH cells), were found throughout the entire extent of the LHRH neuronal field in both sexes. A sex difference in the number of LHRH cells anterior to the organum vasculosum lamina terminalis/preoptic area (a 20% greater number in this region in females) was observed. This sex difference was present throughout development and was not limited to one specific LHRH cell subtype. These data are consistent with the hypothesis that the LHRH system is established early in postnatal development, and that the signal for reproductive maturation is related to the development of synaptic inputs to the LHRH cells.