Sexual dimorphisms have been identified in the brains of many vertebrates. The adult musk shrew brain contains a sexually dimorphic nucleus of aromatase (AROM) immunoreactive (ir) neurons within the medial preoptic area (mPOA). This cell group is larger and contains more neurons in males than in females. We examined the effects of perinatal hormone manipulations on the organization of this nucleus. Administration of testosterone (T) to female pups (days 1–5 postnatal) increased the size of the AROM-ir nucleus. Males castrated on postnatal day I had a smaller AROM-ir nucleus as adults than did males that remained gonadally intact during development. These data show that T and/or its estrogenic metabolite act during an early critical period to organize the development of AROM-ir neurons in the mPOA.