A new device (called a hemodynamometer) employing the principle of the CO2 rebreathing method has been developed for the noninvasive automatic measurement of pulmonary blood flow (cardiac output = CO). The device can also measure oxygen consumption (VO2) and indirect blood pressure and record electrocardiographs (heart rate) at the same time and can automatically calculate total peripheral vascular resistance (TPR) by means of a computer. In this study, the hemodynamometer was used in 28 normotensive subjects and 42 patients with essential hypertension (27 with WHO stage I and 15 with WHO stage II) to evaluate hemodynamics during rest or ergometer exercise. To evaluate accuracy, the dye-dilution (cuvette) method was simultaneously employed in some subjects. The correlation coefficient (r) between CO measured by means of the hemodynamometer and CO measured by the cuvette method was 0.96 (n = 39) with a differential of 0.08 +/- 0.53 l/min and reproducibility was good (r = 0.88, n = 70). The average CO index for normotensive subjects at rest was 3.22 +/- 0.37 l/min/m2 (VO2 = 210 +/- 30 ml/min/m2) and agreed with previously reported values. A significant difference was observed between the hemodynamics (CO and TPR indices) of the WHO-I group and those of the WHO-II group during rest or mild exercise (25-watt load). Because it can measure CO and TPR easily and noninvasively, the hemodynamometer is considered useful for evaluating hemodynamic changes in hypertensive patients.