The effects of intravenous adenosine and adenosine triphosphate (ATP) were studied in a double-blind randomized study during 68 episodes of supraventricular tachycardia in 39 patients. Adenosine restored sinus rhythm in 20 patients (25 of 27 episodes) and produced atrioventricular block to reveal atrial arrhythmias in nine. ATP restored sinus rhythm in 17 patients (22 of 25 episodes) and revealed atrial tachyarrhythmias in six. In patients receiving both compounds, the effective dosage of adenosine was 3.8 mg and of ATP it was 6.6 mg (p less than 0.05), suggesting molar equipotency. Transient side effects were common, occurring in 81% of episodes with adenosine and in 94% with ATP. Symptom scores (0 to 10) were not significantly different (median score for adenosine was 5, for ATP it was 6). Adenosine and ATP were equally effective for the diagnosis and treatment of supraventricular tachycardias and the incidence and severity of side effects were similar. Adenosine has the advantage of being more stable.