The pineal organ of the house sparrow, Passer domesticus, is essential for persistence of the circadian locomotor rhythm in constant conditions. Upon removal of the pineal body, activity becomes arrhythmic. However, pinealectomy does not abolish the rhythm of locomotor activity in birds exposed to light-dark cycles. Pinealectomized birds are entrained by light cycles in much the same manner as are normal birds. Our data demonstrate that the pineal organ is a crucial component of the endogenous time-measuring system of the sparrow.