Female rats were lesioned in the suprachiasmatic nuclei (SCN) electrolytically and treated with methamphetamine. The SCN lesions abolished the circadian locomotor rhythm completely. When methamphetamine was administered in the drinking water, robust rhythmicities in locomotor activity appeared in the SCN lesioned rats, which did not entrain to the 24 hr light cycle. The period of the activity rhythm was dose-dependent; the lower the concentration of methamphetamine was, the shorter the period of the rhythm became. When rats were treated with 0.005% methamphetamine, the mean period was 26.4 hours. In addition, activity time (alpha) became shorter, rest time (rho) longer and alpha/rho ratio lower, when methamphetamine concentration was decreased. After methamphetamine withdrawal, the rhythmicity disappeared and locomotor activity became aperiodic again. When methamphetamine was administered continuously by means of an osmotic minipump, similar rhythmicities appeared in locomotor activity of the SCN lesioned rats. It is concluded that methamphetamine manifests an activity rhythm whose period is in the circadian range. The rhythmicity is independent of the SCN and is not entrained by the light-dark cycle.