Entrained and free-running rhythms of wheel-running activity were recorded in male BALB/cByJ mice with electrolytic lesions of the suprachiasmatic nucleus (SCN), site of a circadian pacemaker in mammals. Complete ablation of the nucleus abolished the circadian locomotor rhythm; in some cases, wheel-running was synchronized by a light-dark cycle, but the phase relationship of this activity to the cycle was often aberrant. Unilateral lesions or those missing the SCN did not eliminate rhythmicity.