1. Antipyrine (1000 mg orally) clearance was studied 3 days before treatment with either atenolol (50 mg twice daily), metoprolol (100 mg twice daily), propranolol (80 mg twice daily) or placebo, and at day 5 and 18 during treatment. Phenytoin (100 mg intravenously) clearance was measured on days 0, 7 and 21 during treatment. 2. Antipyrine clearance was decreased by about 20% after 5 days of treatment with either propranolol or atenolol and this decrease persisted after 18 days of treatment. Antipyrine clearance did not change during treatment with either metoprolol or placebo. Phenytoin clearance did not change during any of the treatments. 1989 The British Pharmacological Society