The effect of oral sotalol on the heart rate-corrected AT interval (QTc) was studied in 33 hypertensive patients. Sotalol given once daily in doses of 160 to 640 mg prolonged in QTc interval in a concentration-dependent manner by up to 150 msec (P less than 0.001) over presotalol levels. The prolongation did not correlate with the initial length of the QTc interval. In seven patients with sotalol-prolonged QTc interval, the withdrawal of sotalol for 3 days shortened the interval to nearly its original length. The effect of sotalol of PQ and QRS times was minimal. Sotalol seems to differ from other beta antagonists in having clear amiodarone-like effects on the action potential of the heart after short- and long-term administration. The measuring of the QTc interval is recommended if high school concentrations are expected, since the risk of cardiac arrhythmias may increase.