The chronotropic and dromotropic effects of the intra-atrial administration of 0.97, 1.93 and 2.90µM/kg Striadyne, the pharmaceutical form of adenosine triphosphate (ATP) for clinical use, ATP, and adenosine, were compared in 13 anesthetized dogs. Striadyne, ATP and adenosine exerted transient dose-dependent negative chronotropic and dromotropic effects. There was no significant difference between the electrophysiologic effects of Striadyne and ATP which were significantly more pronounced than those of adenosine. Atro-pine (0.2 mg/kg) significantly attenuated the electrophysiologic effects of 2.90 µM/kg Striadyne and ATP but not those of adenosine. It is concluded that Striadyne and ATP have similar electrophysiologic effects which are more pronounced than those of adenosine mainly due to vagal involvement in their mechanism of action.