Acute hemodynamics of pimobendan were compared to captopril in a crossover trial in patients with chronic heart failure (NYHA II-III). Heart failure had been stabilized by conventional therapy with diuretics and digitalis for more than 2 weeks. Patients receiving vasodilators were excluded. The hemodynamics were analyzed using a Swan-Ganz catheter at the bedside during drug administration. Following an intravenous injection of 2.5 mg of pimobendan, there was a significant increase in heart rate and decrease in mean pulmonary artery pressure, total pulmonary resistance, mean arterial pressure, systemic vascular resistance and mean right atrial pressure 2 hours after the injection. Captopril (12.5 mg, orally) significantly decreased mean arterial pressure, systemic vascular resistance and double product 2 hours after administration. In this study, the inotropic effect was evaluated through the relation between the stroke volume index and diastolic pulmonary artery pressure, and also between the stroke volume index and mean arterial pressure. Although decreases of diastolic pulmonary artery pressure and mean arterial pressure were seen with both drugs, the differences in stroke volume index were not significant. In comparison with captopril, the acute hemodynamics of pimobendan are characterized as follows: 1) the systemic arteriovasodilating effects of the two drugs were equal, 2) the pulmonary arteriovasodilating effect of pimobendan was marked, 3) a venodilating effect, documented through a decrease of mean right atrial pressure, was seen only with pimobendan. This study concluded that pimobendan is a stronger arterio-venodilator than captopril.