The understanding of brain plasticity is an important challenge facing the neurosciences today. Brain plasticity refers to the fact that experiences of various sorts can lead to enduring changes in the structure and function of the adult nervous system. Investigating how these changes are regulated is essential if clinical brain interventions are to have a scientific underpinning. There are a variety of animal models of brain plasticity available, often involving responses to brain damage or neurodegenerative diseases. Another class of models investigates brain changes that occur naturally in response to salient environmental stimuli such as those regulating the timing of seasonal breeding (e.g. photoperiod). Seasonal changes in the brain of songbirds, such as canaries (Serinus canaria), are an example of such changes. The volume of brain nuclei involved in vocal control change seasonally and testoterone can promote neuronal growth and even modulate the occurrence of neurogenesis. However, the complete causal link from photoperiodic changes to the modification of morphology has not been established. Furthermore, testosterone is not equally effective in inducing changes in brain morphology at all times of the year and afferent neural input can influence steroid action. This proposal consists of five sections; in section I the goal is to characterize the noradrenergic innervation of the vocal control nuclei [Exps 1-4] as a necessary prelude for subsequent functional studies of the role played by this afferent input in the modulation of seasonal changes in the brain. Section II concerns the investigation of the photoperiodic and endocrine control of plasticity in the canary vocal control circuit [exps 5-6]. Physiological reasons for the variation in effectiveness of testosterone will be investigated in section III [Exps 7-8]. Section IV will investigate whether the noradrenergic inputs to vocal control nuclei change as a function of photoperiodic or hormonal manipulation [Exps 9-11] and section V is devoted to the investigation of whether seasonal/testosterone-induced changes in the orphology of the bovcal control nuclei are dependent on the presence of noradrenergic afferent inputs [Exp 12]. The goal of section VI [Exp 13] is to investigate whether seasonsal/testosterone-induced changes in the responsiveness of the neural substrate mediating steroid action ni the brain are dependent on the presence of noradrenergic inputs. Seasonal changes in the songbird brain provide a unique model system to study how naturally occurring stimuli can modify brain morphology.