The effect of cetamolol (an investigational cardioselective beta blocker with intrinsic sympathomimetic activity) on the hypokalemic response to epinephrine infusions in normal subjects was evaluated and compared with placebo and two other beta-adrenergic blocking drugs. After two daily doses of cetamolol 15 mg, atenolol (a cardioselective beta blocker) 50 mg; a long-acting propranolol preparation (a nonselective beta blocker) 80 mg; or placebo, 12 men (mean age, 26.7 years) were infused with epinephrine. The resulting average plasma epinephrine level was 1123 pg/mL, whereas average baseline serum potassium levels for the four treatment groups ranged from 3.94 to 4.07 mEq/L. Epinephrine-induced hypokalemia occurred in the placebo group (maximum potassium decrease of 1.00 mEq/L) and in the atenolol group (maximum potassium decrease of 0.59 mEq/L); potassium levels did not decrease but rose slightly in subjects receiving cetamolol or propranolol. Subjects treated with placebo or atenolol also demonstrated statistically significant prolongation of the QTc interval (0.039 seconds with placebo; 0.023 seconds with atenolol) and frequently developed T-wave flattening and U-wave appearance. After pretreatment with cetamolol or propranolol, however, the QTc interval was unaffected, T-wave abnormalities did not occur, and U waves appeared only rarely. The results of this study indicate that cetamolol blocks epinephrine-induced hypokalemia and associated electrocardiographic changes.