In a randomized, placebo-controlled, double-blind study, 12 patients with congestive heart failure (New York Heart Association class II) were successively treated for 1 week each with placebo, 40 mg famotidine, and 1000 mg quinidine. On the seventh treatment day, heart rate, blood pressure, systolic time intervals, impedance cardiography, and Doppler ultrasound were measured. Heart rate and blood pressure were not markedly altered by either drug. By contrast, quinidine and famotidine significantly decreased stroke volume and cardiac output in impedance cardiography and Doppler ultrasound (p less than 0.05). A high correlation between both noninvasive methods was found for cardiac output and stroke volume (r = 0.93 and r = 0.97 for stroke volume 1 1/2 hours after quinidine and placebo administration, respectively). In addition, the mechanocardiographic ratio of the preejection period to the left ventricular ejection time in systolic time intervals increased significantly 1 1/2 and 3 hours after administration of quinidine and famotidine (p less than 0.05). In conclusion, both quinidine and famotidine exert similar negative effects on cardiac performance, with no significant differences in hemodynamic parameters emerging between the antiarrhythmic agent and the H2-receptor antagonist.