Basic haemodynamic parameters were measured in 58 men in various stages of essential hypertension, 18 patients with hypertensive form of chronic glomerulonephritis, and 23 practically healthy persons during graded exercise in the supine position on a bicycle ergometer for 30 minutes. During exercise, the systolic pressure rose in all persons investigated, whereas the diastolic pressure markedly increased only in patients with arterial hypertension. The cardiac index increased, according to the intensity of the exercise equally in the healthy persons and in patients in early stages of essential hypertension; a lesser increase in the cardiac index was observed in patients in late stages of hypertension and in those with chronic glomerulonephritis. The increase in the cardiac index during exercise is essentially due to an increase in the heart rate; the stroke index increases only slightly, and in later stages of essential hypertension even decreases. The total peripheral resistance diminishes during exercise, but less so in patients with arterial hypertension in whom it is distinctly higher than in healthy persons. The circulating blood volume decreases during exercise in consequence of a decrease in plasma volume, whereas the haematocrit value increases. Renal blood flow and glomerular filtration decrease during exercise, both in healthy untrained persons and in patients with arterial hypertension. In patients in late stages of essential hypertension and in those with glomerulonephritis, the above parameters decrease more markedly and at lower exercise intensity than in healthy persons.