We evaluated the hemodynamic effects of combined administration of metoprolol and tocainide in nine patients with uncomplicated acute myocardial infarction. After 24 h of treatment with oral metoprolol (50 mg q.8 h), intravenous tocainide was given (750 mg over 15 min), followed by oral administration of both drugs (50 mg metoprolol q.8 h; 800 mg tocainide, then 400 mg q.8 h). Hemodynamic variables were measured before and after 25 h of oral metoprolol treatment, immediately after intravenous tocainide (at 15 and 30 min), and then during continued oral treatment with both drugs (at 2 h 15 min and 25 h 45 min). Following injection of tocainide, cardiac index changed significantly (2.7-2.3 L/min . m2; p less than 0.01), but the increases in pulmonary wedge pressure (9-13 mm Hg) and total systemic resistance (15.9-18.1 IU) were not significant. During the continued oral treatment with both metoprolol and tocainide no significant hemodynamic interaction was observed. We conclude that the injection of tocainide can slightly and transiently depress left ventricular function in patients treated with metoprolol.