The postnatal development of the luteinizing hormone-releasing hormone (LHRH) system in male and female rats was investigated using light microscopic immunocytochemistry. An identical number of immunoreactive LHRH cells (about 1,300) was observed for males and females throughout development. Two LHRH cell types, based on morphological appearance, were present in both sexes: smooth LHRH cells and LHRH cells with spine-like processes (irregular LHRH cells). In both sexes, the number of smooth LHRH cells decreases and the number of irregular LHRH cells increases during postnatal life, stabilizing shortly after puberty. We propose that smooth LHRH cells transform into irregular LHRH cells during the development of reproductive maturation.