Blood pressure, cardiac output, plasma volume, renin, and aldosterone were measured in 13 patients with essential hypertension on placebo and after 1, 4, and 12 wk on hydrochlorothiazide 100 mg daily. In 9 patients the same variables were also measured after 24 and 36 wk. Hydrochlorothiazide lowered mean arterial pressure (p less than 0.01). Cardiac output was reduced after 4 and 12 wk of treatment, followed by a return to placebo levels. Stroke volume changed in the same way but heart rate and total peripheral resistance did not differ from placebo values. Plasma volume was reduced after 1 and 24 wk. Renin was permanently elevated (p less than 0.01), but aldosterone rose only during the first 12 wk of treatment. A comparison between responders (greater than 10% fall in mean arterial pressure) and nonresponders (less than 10% fall) revealed different hemodynamic patterns. In responders the initial fall in cardiac output was followed by a return to pretreatment levels, whereas in nonresponders it was permanently reduced. Consequently, total peripheral resistance was lowered only in responders. Nonresponders tended to show a greater degree of plasma volume depletion and greater stimulation of renin and aldosterone, which probably contributed to elevated peripheral resistance. It is concluded that changes in cardiac output are unlikely to be of decisive importance in the ultimate reduction of peripheral resistance in responders to thiazide therapy.