1. Tienilic acid and hydrochlorothiazide were evaluated in a double-blind trial in order to investigate their antihypertensive and metabolic effects. 2. After 5 weeks, the decreases in blood pressure and the changes in plasma or serum electrolytes, urea, creatinine, glucose, cholesterol and triglycerides, and in creatinine clearance, did not differ in the two groups of patients. 3. In patients taking tienilic acid a significant decrease in serum uric acid and an increase in urate clearance was observed, whereas in patients receiving hydrochlorothiazide a slight increase in serum uric acid, with no modification of urate clearance, occurred. 4. The diuretic and antihypertensive actions of tienilic acid and hydrochlorothiazide are very similar. The uricosuric/hypouricaemic effect of tienilic acid could assume clinical relevance in long-term therapy of hypertensive patients.