The ovine sexually dimorphic nucleus (oSDN) is characterized by high levels of aromatase mRNA expression which can be used to delineate its boundaries. The volume of the oSDN is approximately 2 to 3-fold larger in rams that mate with ewes (female-oriented rams) than in rams that mate with other rams (male-oriented rams) and ewes. The sex difference in oSDN volume is present in late gestation fetuses and can be eliminated before birth by exposing genetic females to exogenous testosterone during midgestation, suggesting that early exposure to androgen masculinizes volume of the oSDN. The present study was performed to determine whether differences in oSDN volume are influenced by the adult hormonal environment. Adult rams, behaviorally characterized as female-oriented or male-oriented, and ewes were gonadectomized and treated with subcutaneous implants of testosterone to achieve physiologic concentrations of serum testosterone. Three weeks after implant placement brain tissue was prepared for histological assessment of oSDN volume using in situ hybridization for detection of aromatase mRNA expression. Quantitative analysis revealed that despite similar serum testosterone levels among the groups, the volume of the oSDN was greater in female-oriented rams than in male-oriented rams and ewes (P<0.05). Differences in oSDN volume were specific and not reflective of differences in preoptic area height or brain size. These results suggest that differences in the size of the oSDN in adult sheep were not influenced by adult exposure to testosterone.