We studied the hemodynamic effects of the selective beta 1-adrenoceptor agonist prenalterol in healthy subjects before and after intravenous infusion of the nonselective beta-adrenoceptor blocker propranolol or the beta 1-selective adrenoceptor blocker metoprolol. Intravenous infusions of 0.1, 0.25, and 0.5 mg prenalterol induced a dose-dependent decrease in total electromechanical systole and pre-injection period, whereas left ventricular ejection time remained unchanged. Stoke volume as determined by impedance cardiography was not changed, but a dose-dependent increase in cardiac output due to an increase in heart rate was observed. Systolic blood pressure was significantly increased, but diastolic blood pressure remained unchanged. After intravenous administration of either 10 mg propranolol or 20 mg metoprolol, prenalterol was infused in a 10 times higher dose than before. The hemodynamic effects were similar to those observed before the beta-blockers. Whether propranolol or metoprolol was administered did not influence the effects induced by prenalterol. It is suggested that prenalterol might be a useful drug to counteract undesired hemodynamic effects of beta-adrenoceptor blockers.