18 patients with angina pectoris participated in a double blind trial with atenolol (100 mg and 200 mg once daily, or 100 mg twice daily) and propranolol (80 mg twice daily). The number of anginal attacks (NAP), the number of days free of pain (NAFT), consumption of sublingual nitroglycerin (NNT) and bicycle ergometry data (EFE) were recorded. Atenolol given in a dose of 100 g twice daily significantly reduced NAP and NNT as compared with the other dose schedules for atenolol and propranolol. There was, however, no difference between NAFT and EFE under any of the treatment schedules mentioned above. Only with 100 mg atenolol twice daily was it possible to reduce heart rate at rest and immediately after exercise testing, and also diastolic blood pressure (at rest, upright and after stress testing). In spite of the long plasma T 1/2 (= 24 hours) reported by others, atenolol given twice daily seems to be the most effective schedule. It is concluded that atenolol (100 mg twice daily) has a more potent anti-anginal effect than propranolol (80 mg twice daily). In addition, atenolol has the advantage of being cardioselective.