Biochemical, neural, endocrine and behavioral rhythms play a central role in many different aspects of the reproductive process in animals. These rhythms provide the basis for temporal organization of the reproductive functions of individual animals. In this paper, I introduce our results about the relationship of biological clock to reproductive function, analysis of mechanisms in biological clock, and manipulation of circadian rhythms. Main contents are as follows: 1) The ovulation is induced between 14 and 16 hr after stimulation dissociating entirely with solar hours in light estrous rat. The effectiveness of the cervical stimulation for induction of ovulation in light estrous rats is related to not only the duration of light estrus but also the time after transfer to LL. 2) Both nocturnal and diurnal prolactin surges are controlled by a circadian system. 3) Pseudopregnancy is induced by a single injection of progesterone at a special time. 4) The time of parturition and of pre-partum progesterone decrease is closely associated with an endogenous circadian system. 5) Food restriction is more potent zeitgaber than light in circadian adrenocortical rhythms. 6) The circadian rhythms of blinded rat pups is entrained by the nursing dam. 7) Social cues have a profound effect on the synchronization of the free-running period of adrenocortical rhythm. 8) Carbachol, an acetylcholine agonist, shortens the free-running period of circadian activity rhythm. 9) Melatonin accelerates the re-entrainment of the circadian rhythm to shifted LD cycle. 10) Light signals increase the acetylcholine concentration in suprachiasmatic nucleus, but not the other control site. 11) The cAMP levels in rat suprachiasmatic nucleus show an endogenous circadian rhythm. 12) Long term cultured neuron from rat suprachiasmatic nucleus retain the capacity for circadian oscillation of vasopressin release. 13) Inhibitors of protein synthesis phase shift the circadian clock in chick pineal cells.