This project uses neuroanatomical and behavioral methods to investigate the role of the forebrain in postnatal development. During the second week of life, the golden hamsters, an altricial mammal, loses its primitive thermotaxic mode of responding to the environment and develops many complex species-specific behavior patterns. Forebrain lesions delay this behavioral shift suggesting that inhibition of thermal behavior reflects the maturation of descending influences from the forebrain. The proposed experiments will use the lesion method to identify brain tracts responsible for the decline in thermal orientation and evaluate maturational changes in these fiber tracts using degeneration methods. The amount of long lasting degeneration which is seen in forebrain projections increases dramatically during the second week of life. This provides a convenient method of measuring the maturation of fiber tracts whose functional state can be assessed following their transection. This method may provide an opportunity for correlating maturational changes in behavior with specific developmental changes in brain connectivity. If successful this method would have many applications in studies of neural and behavioral development.