Chronotropic effects of 6 coronary vasodilators, carbochromen, dilazep, diltiazem, dipyridamole, nifedipine and verapamil, were compared by close-arterial administration to the isolated, blood-perfused sino-atrial node preparation of the dog. All drugs produced a dose-related decrease in sinus rate, and the order of negative chronotropic potency determined on a weight basis was as follows: (formula: see text). Higher doses of these drugs except dipyridamole caused atrial standstill. The order of potency causing atrial standstill determined on a weight basis was as follows: (formula: see text). Dipyridamole in lower doses produced a negative chronotropic effect alone, but with higher doses a positive chronotropic effect preceded a negative one. Carbochromen in a wide range of doses produced a triphasic response consisting of initial brief positive, succeeding main negative and final gradually-developing slight positive components.