Before and after hypothiazide and oxodoline (chlortalidone) given in various dosages, hemodynamic parameters were examined in 132 patients with Stage II hypertensive disease at rest, in the orthostatic position, and during exercise testing on a bicycle ergometer. With either drug, a significant resting fall in systolic BP started at 25 mg, in diastolic BP at 50 mg. The hemodynamic mechanisms by which the diuretics exerted antihypertensive action were decreases in stroke index and cardiac index while specific peripheral resistance remained unchanged or slightly increased. Hypothiazide and oxodoline produced no change in orthostatic circulatory response. During bicycle ergometer testing, the absolute systolic BP value was significantly less only when hypothiazide was given in a dise of 100 mg, oxodoline--50 and 100 mg and only when exercise intensities were low (25 W). A significant drop in diastolic BP occurred only at 100 mg of oxodoline and only at low exercise intensities. The diuretics were found to have no pronounced action on the hemodynamic profile during physical exercise.