1. A cross-over study was done in twenty patients with hypertension to compare the effects of chlorothiazide (0·5 g twice daily) and metolazone (5 mg daily) in combination with other anti-hypertensive agents.
2. Compared with absence of diuretic therapy, chlorothiazide and metolazone both produced significantly lower blood pressures.
3. Blood pressures on metolazone tended to be lower than on chlorothiazide but this difference was not statistically significant.
4. Both diuretics significantly lowered serum potassium concentrations and total body potassium to a similar degree, but the serum potassium did not fall below the normal range in any patient and no potassium supplements were needed. No electrocardiographic changes suggestive of hypokalaemia were noted.
5. Small but significant increases in serum bicarbonate, calcium, urea and uric acid were observed with both diuretics.
6. Patient acceptance was excellent and no adverse effects were encountered.