The research project investigates the cellular basis for developmental shifts in responsiveness to endocrine and behavioral effects of testosterone during puberty in males. The hypotheses to be tested include: 1) whether changes in responsiveness to testosterone are mediated by quantitative changes in nuclear retention of steroid in target neurons or differential intraneuronal steroid metabolism; 2) whether the site of action of testosterone negative feedback on gonadotropin-releasing hormone secretion is at the level of neuronal cell bodies or terminals; and 3) whether endocrine and behavioral effects of testosterone are mediated by separate populations of steroid receptive neurons. The experiments employ steroid autoradiography, tract-tracing, immunocytochemistry, and combinations of these techniques.