The administration of 160 mg of propranolol during pregnancy, labor, and delivery was associated with profound hypoglycemia and respiratory depression in a newborn infant. The neonate's plasma propranolol level rose from 40 ng/ml at the time of birth to 90 ng/ml four hours later. This increase in plasma propranolol concentration might be due to redistribution of the drug in the neonate as well as to different elimination mechanisms than in adults. The elevated propranolol level four hours after delivery was not associated with any signs or symptoms of drug toxicity, but drug effect was apparent on the electrocardiogram. The administration of propranolol during pregnancy in doses capable of producing therapeutic maternal blood levels may be dangerous to the neonate.