1. Plasma volume, 24 h urine sodium and total body potassium were similar among 89 previously untreated patients with mild hypertension and 89 control subjects matched for age and sex.
2. Total body potassium correlated with renin activity in both hypertensive patients and controls.
3. Renin activity and total body potassium fell significantly with age in both groups.
4. The relationship between renin and total body potassium was no longer present when the effects of age were allowed for in a partial correlation.