Quazinone (also known as Ro 13-6438 ) is a cardiotonic and vasodilator drug which was developed and marketed in the 1980s for the treatment of heart disease. The positive inotropic response to Quazinone of the isolated guinea pig papillary muscle was accompanied by inhibition of myocardial phosphodiesterase (PDE) activity and elevation of intracellular cyclic AMP (cAMP) levels Quazinone had no effect on Na+, K+-stimulated or Ca2+-stimulated ATPase activity and did not influence the rate of calcium uptake in cardiac membrane vesicles. Quazinone caused a concentration-dependent increase in the upstroke velocity, overshoot, and duration of slow action potentials evoked in partially depolarized papillary muscles. Pretreatment of guinea pigs with reserpine did not prevent the effects of Quazinone on slow action potentials but slightly decreased its positive inotropic activity. In clinical trials, Quazinone induces dose-dependent hemodynamic changes, an increase in cardiac index combined with decreases in pulmonary capillary wedge pressure and systemic and pulmonary arterial pressures.