Serotonin N-acetyltransferase activity in the pineal gland and running acitvity of rats were measured under an ultradian lighting schedule (light/dark 6:6). When rats were moved from a diurnal lighting condition to the ultradian conditions, N-acetyltransferase activity showed a circadian rhythm, increasing once a day. N-acetyltransferase activity in the pups born and raised under the ultradian lighting conditions also exhibited a circadian change, the phase of which coincided with that of their mothers. When pups were raised by a foster mother with an inverted rhythmic phase from that of the original mother, the phase of the rhythm in N-acetyltransferase activity of the pups synchronized with that of the foster mother. When pups were separated from their mothers for 12 h/day, the circadian increase of N-acetyltransferase activity appeared during the dark period when they were separated from their mothers. The circadian rhythms of running acitvity were in phase with those of N-acetyltransferase activity in the pineal gland.