Plasma renin activity and the relationship of renin activity and sodium excretion were analyzed in 50 untreated essential hypertensives classified according to their erythrocyte Na+ transport abnormalities. Renin activity was inappropriately high in 7 patients with increased activity of Na+,Li+ countertransport, inappropriately low in 12 patients with decreased activity of Na+,K+,Cl- cotransport and normal in 31 patients without either Na+ transport abnormalities. These results suggest that there are correspondences between the activity of the renin-angiotensin system and the main Na+ transport abnormalities present in erythrocytes of patients with essential hypertension.