SUMMARY The interrelationships of arterial pressure, plasma volume (PV), and plasma renin activity were studied in 152 consecutive male patients with uncomplicated essential hypertension. Of these, 22 (17 white and 5 black) subjects had normal plasma volumes and because of tbe small number were not included in the analysis of results. Tbe remaining 130 (35 black and 95 white) patients were classified as having either expanded or contracted plasma volume. A higher percentage of black (43%) than white (21%) subjects were volume expanded (PV > 19 ml/cm) and a lower percentage of blacks (57%) than whites (79%) were volume contracted (PV < 17 ml/cm). There was no significant difference in mean arterial pressure and plasma renin activity between tbe volume expanded and contracted black patients. In contrast, the white patients with contracted plasma volume had significantly higher arterial pressures (p < 0.05) and plasma renin activity (p < 0.001) than those with expanded plasma volume. More blacks than whites had low plasma reoin activity and did not manifest the inverse relationship of plasma renin activity to plasma volume as did the whites. These data conflnn and extend previous observations that the relationship between plasma volume and plasma renin activity (PRA) in the male patient with essential hypertension seems to differ between the black and white race. Efforts to explain tbe low PRA in black patients might be best directed toward those patients with suppressed PRA and with contracted intravascular volume.