Feeding and drinking behavior of rats maintained in constant light were recorded before, during and after feeding schedules with periods lying within or outside the range of circadian entrainment. Regardless of period, all schedules immediately resulted in the partial or complete synchronization of drinking behavior, but failed to entrain the free-running circadian feeding and drinking rhythms. This indicates that drinking can be passively driven by periodic access to food. However, other results suggested that a separate circadian system was