Propranolol serum levels during a 24-hr period were determined every 2 hr in 9 hospitalized patients with angina pectoris after oral administration of 40 mg of propranolol 3 times a day. After the first, second, and third tablets the mean maximum serum propranolol concentrations averaged 118 ± 71 ng Iml, 134 ± 97 ng/ml, and 118 ± 94 ng/ml and the mean minimum concentrations averaged 21 ± 18 nglml, 45 ± 25 ng/ml, and 54 ± 34 nglml (±SD), respectively. These data show a very wide inter- and intraindividual variation in serum propranolol levels. No relationship was found between serum level and blood pressure or dose (related to body weight).