Abstract: Objective To investigate the endogenous circadian expression of the Arylalkylamine N-acetyltransferase（NAT）gene in the rat pineal gland.Methods 36 SD rats under constant darkness（DD）for eight weeks were sampled for the pineal gland once every 4 hours in a circadian day.The total RNA was extracted from the samples and the semi quantitative reverse transcription polymerase chain reaction（RT-PCR）was used to determine the temporal changes in mRNA level of the NAT gene during different circadian times（CT）.The data parameters were obtained and analysed by both the cosine function software and the amplitude F test to reveal the circadian rhythm.Results The NAT mRNA level in the pineal gland showed a significantly circadian oscillation（〈i>P