The effect of sound/silence cycles on the motor activity rhythm of the rat was studied. The daily motor activity pattern was studied in two groups of rats of both sexes kept first under DD and later under LL. One group (16 rats) was subjected to a cycle of sound and silence and the other (8 rats) was used as control. The study was performed under two different light regimes, to consider the possibility that a weak zeitgeber (such as sound might be), would exert an effect on the circadian pacemaker. This effect would depend on the stability of the circadian system, which could be modified by the lighting conditions. Results show no statistically significant differences between the two groups, neither in the motor activity patterns nor in the period of the rhythm. It was concluded that sound is not a zeitgeber for the motor activity rhythm of rats.