It has been suggested that the pineal gland has a specific role in the regulation of reproductive functions. Melatonin, secreted by pineal gland, is involved in the control of mammalian reproduction. Previous investigations have show that melatonin reduced the smooth muscle contraction. The purpose of the present study was to investigate the effect of melatonin on the uterine contraction provoked by carbachol. This effect was studied in isolated uterus were taken from Wistar rats pretreated with diethylstilboestrol. Here, we describe the effects of various concentrations of melatonin was found to inhibit the carbachol-induced uterine contraction. Our results show that CE50 of carbachol increment in present to 10(-9) and 10(-6) M/ml of melatonin. The degree of the inhibitory effect of melatonin to concentration of 10(-9) M/ml is most evident that 10(-6) M/ml. Thus, it is concluded that melatonin has an pharmacological inhibition effect on the contraction uterine provoked by carbachol, act as physiological antagonist.