The hemodynamics and the baroreflex function were studied and compared between patients with borderline and established hypertension. 1) As compared to patients with established hypertension, the heart rate, the cardiac output and the left ventricular ejection fraction were higher in patients with borderline hypertension in the young group, but these characteristics were not found in the patients of the middle-aged group. 2) In regard to the cardiac output and the total peripheral resistance, patients with borderline hypertension predominantly showed a high cardiac output, especially in the young group, while patients with established hypertension predominantly showed a high total peripheral resistance, especially in the middle-aged group. 3) At higher and lower blood pressure levels during one day, no difference of hemodynamic pattern was observed in the same subjects. 4) The hemodynamic response to exercise, isoproterenol and noradrenaline was greatest in the borderline hypertensives of the young group. 5) The baroreceptor slope decreased in the order of normal subjects, borderline hypertensives and established hypertensives. It decreased with age in each group. The hemodynamics and the baroreflex function in borderline hypertensives differed slightly between the young and the middle-aged groups. Although the borderline hypertension in the middle-aged group is considered to be a pre-stage of established hypertension, detailed analysis of case histories may be required to assess how borderline hypertension develops into established hypertension in the young group.