The hemodynamic effects of an acute infusion of prenalterol (PN), a new inotropic beta-adrenoceptor agonist, have been evaluated by cardiac catheterization in 10 patients with primary dilated (congestive) cardiomyopathy. A single dose of 20 micrograms/kg was administered over 5 min after basal hemodynamic and angiographic measurements. The administration of prenalterol caused a significant increase in mean cardiac index, from 2.3 to 3.3 l/min/m2 (P less than 0.01) and mean stroke volume, from 47 to 62 ml (P less than 0.01) without a change in heart rate. Mean left ventricular end-diastolic pressure was reduced from 19 to 13 mm Hg (P less than 0.05) and left ventricular dp/dt rose from 902 to 1089 mm Hg/s (P less than 0.01). Stroke work index increased from 27 to 40 g m/m2 (P less than 0.01) and ejection fraction from 31 to 36% (P less than 0.05). Mean blood pressure did not change and the systemic vascular resistance decreased from 24 to 17 RU (P less than 0.01). The favorable effect of prenalterol on left ventricular relaxation was shown by an increase of peak negative left ventricular dp/dt from 946 to 1159 mm Hg/s and by a decrease of the time constant of left ventricular pressure fall from 49 to 39 s. These results demonstrated a positive inotropic effect of prenalterol on patients with diffuse and severely reduced contractility.