Randomized, double-blind. Following one month of withdrawal from all diuretics and repletion with oral potassium and magnesium, the study participants were randomized to two months of treatment with one of six treatment groups: hydrochlorothiazide; hydrochlorothiazide with oral potassium; hydrochlorothiazide with oral potassium and magnesium; hydrochlorothiazide and triamterene; chlorthalidone; or placebo. The main outcome measures were ventricular arrhythmias on 24-hour Holter monitoring and serum and intracellular potassium and magnesium levels.