1. Twenty-three subjects with essential hypertension were followed for a period of up to 7 years. They were untreated during the investigations. 2. Plasma renin concentration was found to decrease temporarily in some subjects. The ultimate change was a gradual rise. In those subjects who suffered myocardial infarction renin tended to rise more sharply. 3. A relationship was established with the rise in renal vascular resistance, which almost invariably occurred over the years.