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Do rats run and respond at a higher rate to run during the dark phase when they are typically more active? To answer this question, Long Evans rats were exposed to a response-initiated variable interval 30-s schedule of wheel-running reinforcement during light and dark cycles. Wheel-running and local lever-pressing rates increased modestly during the dark phase. A second experiment examined the potential role of food-anticipatory activity in this effect by delaying postexperimental session feeding by 6 hr. No increase in wheel-running and lever-pressing rates was observed during the dark phase. This suggests that the effect of light/dark cycle on running and responding for the opportunity to run depended upon foodanticipatory activity. [ABSTRACT FROM AUTHOR]