Plasma levels of propranolol were measured fluorometrically in patients with angina pectoris and in patients admitted to the Coronary Care Unit with acute myocardial infarction. 2. In thirty patients with stable angina pectoris, plasma propranolol levels varied almost linearly with doses between 10 and 120 mg during 6-hourly chronic oral administration. Plasma levels greater than 100 ng/ml produced 70-80% reduction in the tachycardia induced by strenous exercise on a treadmill. 3. In nineteen patients with acute myocardial infarction given oral propranolol, 20 mg 6-hourly, peak as well as trough plasma levels of the drug increased progressively but remained below 100 ng/ml in all except two patients during the first 24 h after their admission to the Coronary Care Unit. 4. The data suggest that the use of low and fixed doses of propranolol may not produce adequate plasma levels or significant beta-adrenoceptor blockade in the early stages of acute myocardial infarction in man.
