The antihypertensive, saluretic and hypokalaemic effects of a small dose of cyclothiazide (2.5 mg daily) were compared with those of a conventional dose of an hydrochlorthiazide-amiloride hydrochloride combination (50 + 5 mg daily). Both preparations were given to 13 patients with mild (WHO I) hypertension in a cross-over manner for six weeks, with an intervening wash-out phase of three weeks. The antihypertensive efficacy of cyclothiazide was well comparable to that of the hydrochlorthiazide-amiloride combination, although cyclothiazide tended to inhibit renal sodium reabsorption less than the combination. Cyclothiazide tended to cause hypokalaemia, apparently due to increased potassium loss, but with the present dosage none of the 13 patients developed marked hypokalaemia (serum potassium less than 3.3 mmol/l). Both drugs led to a comparable increase in serum urate concentration. Neither of the preparations affected creatinine or free-water clearance. The results suggest that even in relatively small doses thiazides effectively decrease blood pressure, and combining thiazides with potassium-sparing diuretics is advantageous only in patients with marked hypokalaemia and its associated risks.