The specific aims of the proposed research are to study the normal anatomical development of the hamster's sensorimotor cortex and its efferent fiber system, the pyramidal tract, and to determine how this developing neural system is affected anatomically and behaviorally by early lesions of the pyramidal tract. The research outlined in this proposal deals specifically with the following aspects of these goals: (1) the pre- and postnatal development of the sensorimotor cortex as revealed by light microscopic tritiated thymidine autoradiography and Nissl methods, (2) light microscopic Nissl studies on the effects of early lesions of pyramidal tract axons on the development and maintenance of the cortical cells of origin, (3) the light microscopic development of the normal topography of the corticospinal neurons and the effects of pyramidal tract lesions on this topography as revealed with retrograde techniques, (4) a light microscopic study of regenerating pyramidal tract axons following early lesions, (5) the postnatal development of the pyramidal tract at the electron microscopic level including a study of the ultrastructural relationship between growing axons and the surrounding substrate, (6) light microscopic studies on redirection of growing pyramidal tract axons after lesions in newborn animals, and (7) behavioral studies on effects of early pyramidal lesions on fine manipulatory movements of the forepaw and the extent of which axonal regeneration and/or sprouting contribute to recovery or maintenance of normal motor behavior.