Running in a novel wheel during the subjective day can shift the circadian activity rhythm of a hamster. The amount of running is thought to be an important variable. We generated a dose-response (activity-phase shift) curve for the amount of wheel running during a 3 h period starting 8 h before normal dark onset in a 14:10 LD cycle. At room temperature (23 degrees C) the relationship was sigmoidal: from 0 to 4000 revolutions resulted in minimal phase advances (up to 50 min). From 4000 to 5000 revolutions the magnitude of the advances increased sharply, and above 5000 revolutions phase advances were asymptotic at about 3 h. The same general relationship held when hamsters were stimulated to be more active in the novel wheel by lowering the ambient temperature to either 11 degrees C or 6 degrees C. However, at these lower temperatures, a significant number of animals did not shift more than the minimal amount of 50 min even though they ran more than 5000 revolutions. This indicates that running per se in a novel wheel was not sufficient to induce phase shifts. Possibly, at room temperature, the amount of wheel running reflects a particular motivational state produced by the rewarding nature of wheel running, although at low ambient temperatures at least some individuals run primarily to meet thermoregulatory needs.