The beneficial action of acetazolamide in the treatment of hypokalemic periodic paralysis was studied in three patients, a woman and two of her sons. All three subjects showed a disturbed electromyogram with a lack of functioning motor units, even when no weakness of the muscle was observed. Bicarbonate-induced alkalosis caused aggravation of the electromyographic pattern. In one subject a paralytic attack was observed during alkalosis, which was abolished with intravenous acetazolamide. A similar beneficial action on previously disturbed electromyographic pattern was observed during ammonium chloride infusion. After one week of treatment with acetazolamide the electromyographic records were found to be almost normal in all three subjects. The effect of acetazolamide on these three patients could not be explained by its action on serum potassium levels, which did not change. It appears that the acidosis caused by acetazolamide has a beneficial effect on the disturbed muscle excitability found in these patients.