Effects of 2-nicotinamidoethyl nitrate (2-NN) on smooth muscle cells of porcine and guinea-pig coronary arteries were investigated by the microelectrode and double sucrose-gap methods and from isometric contractions. In the dose over 2 X 10(-6) M in the guinea-pig coronary artery and over 5 X 10(-6) M in the large and small porcine coronary arteries, the membrane was hyperpolarized and the ionic conductance of the membrane was increased. This hyperpolarization appeared more dominantly in low [K]0. The action was estimated to be due to increase in the K-conductance of the membrane in both species. The spike with an overshoot evoked by electrical stimulation in the presence of tetraethylammonium chloride (TEA) was suppressed by 2-NN, mainly due to hyperpolarization of the membrane and not to suppression of Ca-carriers at the membrane. The contraction evoked by electrical depolarization, acetylcholine (ACh), excess [K]0 or repetitive short pulse stimulation was suppressed in the presence of 2-NN with a concentration similar to that which hyperpolarized the membrane. The suppression of the mechanical response is postulated to be mainly due to hyperpolarization of the membrane and in part to suppression of the Ca-mobilization from the storage sites in the cell. The former seems to be the main action of 2-NN on the coronary arteries in both species. Definite differences on 2-NN actions between the large and small porcine coronary arteries were not observed, but the contraction evoked in the large coronary artery was suppressed slightly more by 2-NN. Vasodilation which occurs with application of 2-NN is discussed in relation to that induced by nitroprusside or nitroglycerin.