?s abstract) The addition of new neurons into an established neural pathway may facilitate behavioral plasticity but also require training to be behaviorally adaptive. In adult songbirds, species differences in the extent of neuronal incorporation correlate with differences in song degradation after adult deafening, and the aim of the proposed experiments is to test further this relationship between neuron addition and behavioral plasticity. First, an antimitotic agent will be used to attenuate neuron addition and then determine if this attenuates behavioral change after deafening. Second, behavioral plasticity after deafening will be prevented by lesioning a specific part of the avian song system, and it will be determined if this manipulation also attenuates the addition of new vocal motor neurons. Third, adult-generated neurons will be labelled with bromodeoxyuridine and in situ hybridization will be used to determine if the expression of NMDA receptor mRNAs in these neurons recapitualtes maturational changes associated with periods of enhanced plasticity during development. These experiments will elucidate how the addition of new neurons in adulthood affects behavior, and will shed light on the consequences of endogenously and exogenously promoted neurogenesis and neuronal recruitment in humans.