There are large sex difference in the structure and function of brain regions controlling song in passerine birds. These sex differences develop under the control of gonadal steroids. The long term objectives are to understand the cellular mechanisms by which gonadal steroids influence the development of these neural circuits that control a complex learned motor pattern. By gaining knowledge of these cellular mechanisms, we will also shed light on the neural mechanisms responsible for song learning and for the limitation of learning to an early critical period. Using castration and various treatments with steroid hormones and antihormones, we will examine which steroids act singly or in combination to alter the morphological development of the song system (numbers and sizes of neurons). Other experiments will measure steroid accumulation and projections of steroid sensitive neurons to clarify possible sites of steroid action in the control of neural development. Using thymidine autoradiography, we will assess when song system neurons undergo their last mitotic division, and manipulate steroid levels to determine if steroids regulate neuron number by altering the processes of proliferation or migration of neurons. Anatomical tract tracing techniques will map the ontogeny of interconnections between song system neurons to suggest possible times at which these neurons can have trophic interactions regulated by steroids.