Twenty patients aged 33 to 69 years with uncomplicated hypertension, no heart disease, and normal stress test results underwent ambulatory ECG monitoring a month after receiving placebo and two and four weeks after hydrochlorothiazide therapy. Serum potassium level (+/- SEM) averaged 4.4 +/- 0.09 mEq/L after the placebo trial and 3.4 +/- 0.07 and 3.0 +/- 0.06 mEq/L after two and four weeks of therapy, respectively. Sixteen patients had no arrhythmias. Four patients had 329 +/- 140 premature ventricular beats (PVBs) while receiving placebo and 341 +/- 203 and 315 +/- 158 PVBs per 24 hours after two and four weeks of therapy, respectively. Thus, patients with uncomplicated hypertension and no arrhythmias before diuretic therapy did not experience arrhythmias as a result of diuretic-induced hypokalemia of one month's duration. Patients with low-grade ventricular ectopy (VE) before therapy did not progress to higher grades of VE after diuretic treatment for four weeks.