Research in my lab concerns the actions of sex hormones in the brain. In mammals, brain tissue, particularly that which is involved in neuroendocrine function and reproductive behavior, contains high levels of cytochrome P450 aromatase. This enzyme converts circulating testosterone into estradiol. Although estradiol is generally considered a female hormone, aromatization seems to constitute a complementary signal pathway for androgen action in certain brain tissues. Estrogens are known to exert profound feedback effects on the brain that regulate reproduction and, more generally, are able to induce synaptic connections and protect neurons from injury. Locally-synthesized estradiol appears to play a role in male physiology that augments effects exerted by circulating testosterone.  Our current research uses sheep to address the question of how testosterone acts during development to organize male-typical features of brain morphology and neurochemistry to produce associated sex-differences in reproductive behaviors.   Sex differences in brain development are important to understand because they underlie robust neurological differences between healthy males and females and contribute to sex biases in the incidence of certain brain diseases and psychiatric conditions.