1. Propranolol, a beta-adrenoceptor blocking drug with local anaesthetic and a direct myocardial depressant action, and MJ 1999, a beta-adrenoceptor blocking drug which has no local anaesthetic or intrinsic sympathomimetic action, were compared for beta-adrenoceptor blocking activity in man.2. Propranolol was 2.67 times more active than MJ 1999 in reducing by 50% the tachycardia produced by the intravenous infusion of isoprenaline in healthy volunteers.3. Propranolol and MJ 1999 intravenously both reduced resting heart rate in the standing position and an exercise tachycardia, but there was no qualitative or quantitative difference between them.4. On oral administration, both propranolol and MJ 1999 reduced resting heart rate and an exercise induced tachycardia; propranolol was only slightly more active than MJ 1999.5. In patients with thyrotoxicosis propranolol was about twice as active as MJ 1999 in reducing the heart rate.