Resveratrol, a natural phytoalexin found in wine, has been suggested to have benefits in preventing cardiovascular diseases. However, the direct effects of resveratrol on the activity of cardiac tissues and its mechanism of action have not been determined. This study examined the effects of resveratrol on the right and left atrium and left papillary muscle isolated from the rat heart. The contractile responses of the right atrium and papillary muscle and the actionpotential from the left atrium were recorded and the effects of resveratrol on these responses were observed. The resting force of the isolated right atrium and the peak developed force of the left papillary muscle were depressed by resveratrol (0.1 nM - 0.1 mM). Exposure to the K$_{ATP}$ channel blocker glyburide (3 ${mu}$M) prevented signifcantly the resveratrol-induced decrease. Resveratrol (0.1 mM) shortened the repolarization phase of action potential recorded from the left atrium and this effect of resveratrol was reversed by glyburide (3 ${mu}$M). These results indicated that resveratrol depressed cardiac muscle contraction and shortened action potential duration probably due to the activation of K$_{ATP}$ channels in the rat heart.