The suprachiasmatic nucleus (SCN) is implicated in the control of circadian rhythms of gonadal function. Although several structures surrounding the SCN are sensitive to the effects of gonadal steroids, similar effects in the SCN remain unclear. For example, there are conflicting data on whether the SCN is sexually differentiated. This study attempted to determine sex differences in the number of SCN cells generated during late gestation, and if testosterone mediates these differences. Pregnant female rats were treated with 5-bromo-2'-deoxyuridine (BrdU; 50 mg/kg) on gestational day 18 (E18), the day when aromatase activity peaks in the developing rat fetus. These animals were also given injections of oil or testosterone propionate (10 mg/0.1 mL peanut oil) from E15 until parturition. Litters were allowed to survive until adulthood and were killed on postnatal day 60 (PN60). Following fixation, brain sections containing the SCN from these rats were processed for BrdU immunocytochemistry. A second set of SCN sections was processed for immunocytochemistry detecting BrdU and some of the cell groups prevalent within the SCN. Data showed that female rats have a higher number of cells labeled with BrdU in the SCN, particularly in the medial and caudal SCN. This sex difference was abolished in animals treated with testosterone during late gestation. Double immunocytochemistry revealed that BrdU-labeled cells were neurons expressing calbindin-D28K, vasoactive intestinal peptide and, to a lesser degree, vasopressin. Our results unveiled a previously unknown effect of gonadal steroids on the developing SCN, which may contribute to the emergence of gender-specific circadian rhythms.