The influence of cardioselective beta blockade on systemic hemodynamics and cardiac performance was assessed in 17 essential hypertensive patients treated with metoprolol (50 to 100 mg twice daily) for 1 month. Eleven patients had a significant reduction of mean arterial pressure (less than 10 mm Hg); the other six patients did not show significant blood pressure change. There was no difference in response of heart rate or cardiac output between the two groups [-14 +/- 1.6 (SE) vs. -9.5 +/- 2.08 bpm, NS, and -0.55 +/- 0.39 vs. 0.80 +/- 0.35 L/min, NS, respectively]. Mean transit time was significantly prolonged in both groups (1.9 +/- 0.56 vs. 1 +/- 0.53 seconds, NS), and changes in ejection fraction in the two groups were not significant. The only significant difference between the two groups was in the response of total peripheral resistance (-1.4 +/- 3.04 vs. +10.2 +/- 3.77 U . m2, p less than 0.05). The reduction in heart rate correlated significantly with control heart rate (r = -0.71, p less than 0.001) and the decreased in cardiac output correlated significantly (r = -0.77, p less than 0.001) and the decrease in cardiac output correlated significantly (r = -0.77, p less than 0.001) with the initial cardiac output. The changes in mean arterial pressure did not correlate with changes in cardiac output (r = 0.116). On the other hand, there was a significant correlation of 0.68 (p less than 0.01) between the changes in mean arterial pressure and the changes in total peripheral resistance. Changes in plasma renin activity (PRA) were not significant in the nonresponders (1.8 +/- 0.9 to 0.6 +/- 0.2 ng/ml, NS), and the reduction of plasma renin activity was significant in the group of responders (5.5 +/- 2.4 to 1.7 +/- 0.8 ng/ml, p less than 0.05). However, there was no correlation between the changes of mean arterial pressure and either initial PRA or the change in PRA. As regards plasma catecholamines (measured in nine patients), there was a tendency to reduction in both responders and nonresponders (411 +/- 84 to 319 +/- 67 ng/L and 562 +/- 141 to 388 +/- 166, respectively); but changes did not reach statistical significance in both groups. It was concluded that metoprolol reduces cardiac output by slowing heart rate; this reduction of cardiac output was not related to decreased cardiac performance. The mechanism of blood pressure reduction seems to be related to the response of systemic resistance to the diminution in cardiac output and not to the decrease in cardiac output per se, inasmuch as the latter occurred in both responders and nonresponders.