A crossover comparison of metoprolol and hydrochlorothiazide has been performed in 20 patients with mild hypertension. Both drugs caused almost identical statistically significant reduction in blood pressure of about 20 mm Hg systolic and 15 mm Hg diastolic. The side effects during active therapy were few and mild, but 5 patients experienced subjective symptoms during the first few days following abrupt withdrawal of metoprolol, namely general malaise, palpitations, headache, sweating and tremor. The symptoms were more pronounced in the standing position and disappeared at once on resumption of beta-blocker therapy, or gradually over 5 - 7 days when placebo tablets were given. In 11 of the 20 patients hydrochlorothiazide produced subnormal serum potassium levels and potassium supplements were given. The serum uric acid level was also significantly increased during hydrochlorothiazide treatment.