Radioimmunoassay for the measurenment of plasma renin activity (PRA) was performed in 43 normal Koreans and 67 patients with essential hypertension. Urinary sodium excretion rates were measured in the concurrently collected 24 hour urine samples as an index of their sodium balance. The results were as follows: 1. There was an inverse correlation between 24 hour sodium excretion and PRA. The normal values of PRA in supine position ranged from 1.0 to 7.0 ng/ml/hr. When 24 hour sodium excretion were between 50 to 150 mEq. PRA in elderly persons tended to be low. 2. Of the 67 patients with essential hypertension, PRA was low in 20 cases (29.9%), normal in 36 (53.7%), and high in 11 cases (16.4%). 3. In the normal and high renin groups, who tended to be younger in ages, blood urea nitrogen was slightly higher than in low renin group, and hypertensive retinopathy was more prevalent in normal and high renin groups. 4. There were 17 cases with cardiovascular complications(12 with cerebrovascular accidents, 5 with heart failure): 6 in low renin group (30%), 9 in normal (25%) and 2 in high renin group (18.2%). There was no evidence that more frequent complications developed in higher renin groups.