The positive inotropic effect of the aqueous extract of Convallaria keiskei (ACK) and the possible mechanisms responsible for this effect were investigated in beating rabbit atria. ACK significantly increased atrial stroke volume, pulse pressure, and cAMP efflux in beating rabbit atria. The effects were not altered by pre-treatment with staurosporine and diltiazem, a non-selective protein kinase inhibitor and an L-type Ca2+ channel blocker, respectively. In addition, ACK markedly increased the K+ concentration in the beating atria-derived perfusate. Convallatoxin, a well-known digitalis-like cardiac glycosidic constituent of ACK, also increased atrial stroke volume and pulse pressure but did not alter the cAMP efflux level. The increases in atrial stroke volume and pulse pressure induced by convallatoxin were not also altered by pre-treatment with diltiazem. These results suggest that the ACK-induced positive inotropic effect in beating rabbit atria may, at least in part, be due to the digitalis-like activity of convallatoxin.