Age-related changes in cardiovascular regulatory mechanisms may affect blood pressure homeostasis during sleep and in the daytime. This study compared systemic hemodynamics during the daytime and sleep between 12 young or middle-aged patients (young, 42.1±13.9 years old, mean±SD, less than 56 years old) and 12 elderly patients with essential hypertension (old, 65.3±2.8, 60 to 70 years old). They were all hospitalized and placed on a diet containing approximately 7 g sodium chloride per day. Intra-arterial blood pressure and electrocardiogram were recorded for 24 hours, and electroencephalogram and electroophthalmogram were recorded during the night with a telemetric method. Cardiac output was measured with patients in the supine position by the cuvette method during the daytime and stage 3 or 4 sleep at night. The averaged 24-hour blood pressure was similar in the two groups (140±2 [SEM]/85±3 mm Hg in the young group and 144±4/81±2 mm Hg in the old group). The reduction in mean blood pressure during sleep was also comparable in both groups (−18±2 in the young group and −20±2mm Hg in the old group). Cardiac index was smaller in the old group than the young group during both the daytime and sleep (daytime, 2.3±0.1 versus 3.2±0.2 [L/min]/m2, P<.01; sleep, 2.1 ±0.1 versus 2.6±0.2 [L/min]/m2, P<.01). The reduction in cardiac index during sleep was greater in the young than the old group (P<.05). Total peripheral vascular resistance index was elevated in the old group compared with the young group (daytime, 47.1±1.8 versus 36.2±2.6 mm Hg · min · m2/L, P<.01; sleep, 41.4±2.4 versus 36.1±2.1, mm Hg · min · m2/L, P<.05). Although the reduction in total peripheral vascular resistance index during sleep was not significant in the young group, it was pronounced in the old group, showing a significant group difference (P<.001). These findings indicate that different hemodynamic components are involved in the reduction of blood pressure during sleep in young and old hypertensive patients.