Hemodynamic changes in supine and upright position and during exercise were studied in 144 men including 37 normal subjects and 107 patients with borderline hypertension. Borderline hypertensives were classified in 4 groups, according to the basal level of heart rate and stroke index. In the first two groups, stroke index was normal, but heart rate was either decreased (Group I) or elevated (Group II): supine total peripheral resistance was superior (Group I; p less than 0.02) or equal (Group II) to normal values; plasma volume was normal or slightly decreased; hemodynamic response to tilt test closely approximated the normal; impaired adaptation of cardiac index and stroke index to exercise was observed in Group II (p less than 0.001). In the other two groups, stroke index was significantly elevated (p less than 0.0001) but heart rate was either decreased (Group III) or increased (Group IV); supine total peripheral resistance values were decreased (p less than 0.001); plasma volume was equally reduced (p less than 0.001); abnormal percentile increase in diastolic arterial pressure (p less than 0.02; p less than 0.001) and total peripheral resistance during tilt test were observed; during exercise, only the patients of Group IV had an impaired adaptation of cardiac index (p less than 0.001) and total peripheral resistance (p less than 0.02). The study provide evidence that in boderline hypertension i) a high basal level of stroke volume is associated with a reduced plasma volume and an imparied adaptation of total peripheral resistance during tilt, ii) a high basal level of heart rate is associated with an impaired adapatation of cardiac output during exercise, and iii) only patients having an increase in both heart rate and stroke volume exhibited a specific hemodynamic pattern including: reduction of plasma volume, impaired adaptation of cardiac output during exercise, impaired adaptation of total peripheral resistance during tilt and exercise.