The sexually dimorphic nucleus of the preoptic area (SDN-POA) is larger in volume in males, is responsive to steroids developmentally, and contains a subpopulation of late-arising neurons that can be specificially labeled with 3H-thymidine on embryonic day 18 (EM). The cytoarchitecture of this region has been described, and one component, the central part of the medial preoptic nucleus (MPNc), shows considerable overlap with the SDN-POA. One goal of the present study was to relate the two by determining if testosterone propionate (TP) exposure perinatally increases MPNc volume and neuronal number, and by characterizing the distribution of the late arising neurons of the SDN-POA with respect to the MPNc. A second goal was to determine if these late-arising neurons are a representative, hormone-sensitive population. Finally, TP exposure was delayed past the time of the endogeneous testosterone surge in males and after the neurons have become postmitotic, to determine if female brain structure could still be sexreversed under these conditions. Pregnant rats were injected on El8 with 3H-thymidine. Daily injections of 2.0 mg TP were given to the mothers starting on either El6 or E20 and continued through birth. The pups were injected daily with 100 pg TP from birth through postnatal day 10. Control rats, from mothers given oil from El6 until birth, were injected with oil from birth through postnatal day 10. Rats were sacrificed at 30 days of age and their brains processed for autoradiography. Neurons and labeled cells were counted in three areas: (1) the MPNc, (2) the region surrounding the MPNc, and (3) a region anterior to the MPNc within the SDN-POA. Labeled cells composed approximately 16% of the neurons in the MPNc and only 3 to 4% in the control regions. Within the MPNc there were more neurons and labeled cells in males than in females. Testosterone treatment, regardless of the starting day, increased MPNc volume and its number of neurons and labeled cells to those of males. These data demonstrate that there is a sex difference in the number of late-arising neurons at 30 days of age, that they are sensitive to steroids during development, and that they are localized to the MPNc. In addition, TP exposure is effective beyond the time of the endogeneous surge in males. Steroid injections in one group of animals were not started until neurons in this region had become postmitotic but still resulted in a sex-reversal, suggesting that testosterone does not modify neurogenesis.