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This study was carried out to further elucidate the effects of adenosine and adenosine-5′-triphosphate (ATP) on atrioventricular (AV) conduction in patients. Adenosine (0.24 mg/kg) and ATP (0.28 mg/kg) were administered intravenously to 37 patients undergoing intracardiac electrophysiologic evaluation. Both adenosine and ATP depressed AV conduction by lengthening the atrial to His bundle (AH) interval. The effects of adenosine and ATP after rapid intravenous bolus administration were fast in onset (15 ± 0.5 and 15 ± 1.5 s, respectively), but transient in duration (10.5 ± 0.5 s for ATP and 17 ± 3 s for adenosine). Although muscarinic blockade with 0.04 mg/kg atropine shortened the AH interval from a control value of 123 ± 12 to 74 ± 4 ms, it did not modify the effects of adenosine or ATP, or both (that is, latency and duration of the effects were not significantly different from before atropine administration).