Recently some researches have established that the melatonin, secreted by pineal gland, may evoke the changes of contractile responses on smooth muscles. We examined the effects of melatonin on the motility of rat pyloric antrum and which mechanism might be involved in the effects. Pyloric antral strips from the stomach of 20 Sprague-Dawley rats were prepared for isometric tension recording in organ bath. The strips were precontracted by acetylcholine and high-KCl solutions. In precontracted conditions the tensions were increased by accumulative application of melatonin ($10^{-8}-10^{-4}$ M) dose-dependently, even in resting states. And the effects were almost disappeared when the concentrations of ACh were over than 10 ${mu}M$. The effects of melatonin were inhibited by pretreatment of 10 mM TEA and/or 10 ${mu}M$ 4-AP and rarely affected by pretreatment of 1 mM TEA, 10 ${mu}M$ glibenclamide and 10 ${mu}M$ verapamil respectively. From these results it is concluded that the contractile responses of smooth muscles of rat pyloric antrum were enhanced by melatonin application and the mechanism might be concerned with the inhibition of some voltage-dependent potassium channels.