Activity-rest patterns displayed by an animal under various circumstances are suggested to result from the combined influences of two virtually identical circadian pacemaker components. Increased output of each component proportionally increases the probability of activity of the animal. Such a dual circadian pacemaker model explains much of the phenomenology of activity-rest records. Under normal light-dark schedules, the two components of the model have a similar phase relationship to the Zeitgeber. Differential synchronization of the components to dawn and dusk is not assumed.