We studied the effects of five beta-adrenoceptor blocking agents (propranolol, atenolol, sotalol, oxprenolol and carteolol) on sinus cycle length (SCL), sinoatrial conduction time (SACT) measured with a constant atrial pacing technique, and atrial developed tension (DT), using isolated blood-perfused canine atria. Except for carteolol, all tested beta-blockers were infused into the sinus node artery at a rate of 1-4 micrograms/min, and carteolol was given continuously at a rate of 0.1-0.4 micrograms/min. Atenolol and sotalol as well as propranolol prolonged SCL and SACT and reduced DT dose-dependently, although propranolol had more potent negative chrono-, dromo- and inotropisms than atenolol and sotalol. Oxprenolol did not change SCL significantly, but lengthened SACT at higher doses and decreased DT in a dose-related manner. Carteolol did not cause any significant changes in the three parameters. The relative potencies on deceleration of SA conduction on the basis of administration of doses which seems to be equipotent as to beta-blocking effect are as follows: propranolol greater than atenolol = sotalol greater than oxprenolol = carteolol. These results suggest that the membrane stabilizing action of beta-blockers may lengthen SACT but that the intrinsic sympathomimetic action of beta-blockers may resist such a prolongation of SACT.