The present study examined whether patients with systemic hypertension have evidence of structural vascular changes, whether such changes can be detected in early stages of hypertension and whether they are reversible with treatment. Hypertensive and normal subjects were studied under conditions of maximal vasodilation in which flow at a given driving pressure was considered to give an index of structural changes in resistance vessels. Thirty-two subjects were separated into 4 groups: 8 with sustained hypertension, 8 with intermittent hypertension, 8 treated hypertensive subjects maintained at systolic pressures of less than 125 mm Hg with drugs for 5 years, and 8 normal subjects. Flow was measured by venous occlusion plethysmography after 10 minutes of ischemia, using a water-filled plethysmograph at 43 degrees C. Arterial blood pressure was measured by the arm cuff method. Transmural pressure, calculated as mean arterial minus external pressure, was varied by imposing varying external hydrostatic pressures. Flow at a transmural pressure of 85 mm Hg was calculated for each subject from the least-mean-square plot of transmural pressure vs flow. Mean flow for normal subjects was 41 ml/100 ml/min and differed significantly from that for sustained hypertensive patients (30 ml/100 ml/min), treated hypertensive patients (33 ml/100 ml/min), and intermittent hypertensive patients (32 ml/100 ml/min) (p less than 0.05). There was no overlap between sustained hypertensives and normal subjects, but half of the treated hypertensive patients were normal.(ABSTRACT TRUNCATED AT 250 WORDS)