In the course of routine screening of beta-adrenoceptor blocking compounds, it was found that a number of compounds did not block the vascular and cardiac effects of isoprenaline in proportion. Moreover, it was also found that the anti-arrhythmic action was not wholly related to the beta-adrenoceptor blocking potency. In particular, the anti-ouabain arrhythmia properties were not dependent upon beta-blocking potency. It was, therefore, possible to select a compound which had both cardioselective beta-adrenoceptor blocking and anti-ouabain arrhythmic activities. The chosen compound was acebutolol. Acebutolol is a cardioselective beta-adrenoceptor blocking agent which has marked anti-arrhythmic against arrhythmia induced by methylchloroform and adrenalin in the cat, and also arrhythmia induced by ouabain in dogs and rabbits. It has less pronounced membrane stabilising properties than propranolol and has no demonstrable sympathomimetic action in normal anaesthetised cats or dogs. Bronchial beta-adrenoceptor blockade has been investigated in anaesthetised cats in which bronchoconstriction is evoked by PGF2 alpha. Isoprenaline prevents the effect of PGF2 alpha and this is restored by pre-treatment with propranolol. Acebutolol is 100 times less active than propranolol in these experiments.