The efficacy of low dosages of diuretics was evaluated in two studies. In one, 62 (48%) of 130 patients became normotensive with 2.5 mg/day of metolazone. In the other, 28 (49%) of 57 patients became normotensive with 25 mg of chlorthalidone, compared with 12 (22%) of 55 patients given placebo. There was a marked variation in blood pressure response and the occurrence of hypokalemia (less than 3.5 mEq/L of potassium) from center to center and within patient groups in both studies. The mean decrease in serum potassium was between 0.5 and 0.6 mEq/L in the metolazone group and 0.44 mEq/L in the chlorthalidone-treated patients. This degree of hypokalemia is only slightly less than that noted when larger dosages of thiazide diuretics are used (0.6 to 0.7 mEq/L). It is concluded that 2.5 mg/day of metolazone or 25 mg/day of chlorthalidone are effective antihypertensive agents but that blood pressure lowering may be inconsistent at these dosage levels. It is reasonable, therefore, to begin diuretic therapy with low dosages, but larger dosages (5 mg of metolazone or 50 mg of chlorthalidone) should be tried before adding another drug or concluding that diuretic therapy is ineffective if an acceptable blood pressure response is not obtained. The degree of hypokalemia that occurs at lower-dose therapy is variable but may be of less clinical significance than that noted with higher dosages of diuretics in some patients.(ABSTRACT TRUNCATED AT 250 WORDS)