The suprachiasmatic nucleus (SCN) of the hypothalamus is considered to be the circadian oscillator (clock) and is entrained by the environmental light/dark cycle. The SCN influences various physiological and behavioral activities. Locomotor activity exhibit a clearly defined circadian rhythmicity. The neuroanatomical substrates subserving the circadian locomotor activity are unknown. Bilateral lesioning of the SCN renders the locomotor activity arrhythmic. Transplantation of hypothalamic tissue containing the SCN into the third ventricle restored circadian locomotor activity in SCN lesioned animals. We hypothesize that the circadian entrainment of locomotor activity by the SCN is mediated via efferent projection fibers to specific neuronal population(s) within the motor system. The proposed three year project is designed to identify the neuroanatomical substrates that communicates SCN generated circadian signals within the motor system.