Adenosine provokes atrial fibrillation (AF) in some patients with paroxysmal supraventricular tachycardia (PSVT). Which patients are more susceptible to develop atrial fibrillation after the administration of adenosine to terminate PSVT is unknown. We prospectively measured atrial action potential duration (APD) at incremental doses of 3, 6, and 12 mg of adenosine at paced cycle lengths (CLs) of 600, 500, and 400 ms in 25 patients. Bolus injection of adenosine decreased APD at 90% repolarization in a dose- and rate-dependent manner. During paced CLs of 600, 500, and 400 ms, decreases of 8%, 13%, and 19% (p < 0.05), respectively, were found after bolus administration of 3 mg of adenosine. After 6 mg of adenosine, the APD shortened by 12%, 19%, (p < 0.05), and 27% (p < 0.01), respectively. After 12 mg of adenosine, the APD shortened by 15%, 27% (p < 0.05), and 38% (p < 0.01), respectively. Transient AF occurred in 4 of 25 (16%) patients, all during paced CLs of 400 ms, and after adenosine 6 mg in one patient and 12 mg in three patients. Adenosine shortens atrial action potential duration in a dose- and rate-dependent manner. Whether patients with faster rates during PSVT and those given higher doses of adenosine are more prone to develop atrial fibrillation remains to be determined.