The inotropic action of prostaglandins PGF_2α, PGD₂ and PGE₂ on isolated mouse left atria was characterized and compared with the positive inotropic action of acetylcholine, which has previously been shown to be mediated by prostaglandins released from the endocardial endothelium. PGF_2α, PGD₂ and PGE₂ produced positive inotropic responses; the time course of the change in contractile force induced by PGF_2α and PGD₂ was about the same as that by acetylcholine, while that by PGE₂ was slower. Fluprostenol and sulprostone, FP and EP receptor agonists, respectively, had positive inotropic effects while BW-245C, a DP receptor agonist, had no effect. AH-6809, a DP receptor antagonist, had no inhibitory effect on the positive inotropic response to PGD₂. Dimethylamiloride, an inhibitor of Na⁺/H⁺ exchange, inhibited the positive inotropic response to PGF_2α, PGD₂ and acetylcholine, but not PGE₂. Fluorometric pH measurement with carboxy-SNARF-1-loaded atrial myocytes revealed no change in intracellular pH on application of PGF_2α. PGF_2α and PGD₂ significantly prolonged the duration of the atrial action potential while PGE₂ had no significant effect. These findings suggest that prostaglandins induce positive inotropic response in mouse atria through FP and EP receptor stimulation and that the former mechanism mediates in part the positive inotropic response to acetylcholine.