Objective: To study the effect of Kaixin San on the rate-limiting enzyme in biosynthesis of melatonin (MT) and pineal body in rat depression model., Method: The unpredictable chronic mild stress was used to establish the rat depression model for 21 days. The rats were divided into the normal control group, the model group, Kaixin San low, medium and high dose groups (KXS 65, 130, 260 mg x kg x d(-1)) and the trazodone group. All groups were administered at 30 min after modeling each day. Rats were sacrificed and the pineal glands were isolated immediately after acquisition tail venous blood at 2:00a. m on the 22nd day. The plasma was analyzed for melatonin content by using a rat metabolic panel Milliplex kit. The pineal glands were analyzed for AANAT and HIOMT mRNA levels by Real-time quantitative PCR and for AANAT and HIOMT activity by a radiometric assay simultaneously., Result: The plasma MT concentration, expression of AANT and HIOMT mRNA, activity of AANAT in rat pineal glands of the model group were significantly lower than the control group (P < 0.05), but the activity of HIOMT showed not change. Compared with the model group, all of Kaixin San groups showed increase in MT concentration in plasma (P <0. 05) , with the medium dose group revealing the highest level. Besides, the medium dose group displayed significant increase in AANAT, HIOMT mRNA level and AANAT activity (P < 0.05), but no increase in HIOMT activity., Conclusion: Kaixin San can regulate AANAT activity of pineal bodyand regulate MT biosynthesis in rat depression model.