Introduction: Melatonin(N-acetyl-5-methoxytryptamine)has marked antioxidant properties,which are important for the optimal function of the peripheral reproductive organs.Although,the melatonin synthetic enzymes,as well as the MT1/2 receptors has been discovered in several ovarian cell compartments indicating that melatonin functions as a local regulator within ovary,the functions of melatonin in preovulatory fluid remain elusive.