Adenosine has been shown to affect both sinus node automaticity and atrioventricular (AV) nodal conduction. The effects of increasing doses of intravenous adenosine were assessed in 46 patients with supraventricular tachyarrhythmias. Adenosine reliably terminated episodes of supraventricular tachycardia in all 16 patients with AV reciprocating tachycardia, in 13 of 13 patients with AV nodal reentrant tachycardia and in 1 of 2 patients with junctional tachycardia with long RP intervals. Adenosine produced transient high grade AV block without any effect on atrial activity in six patients with intraatrial reentrant tachycardia, four patients with atrial flutter, three patients with atrial fibrillation and in single patients with either sinus node reentry or an automatic atrial tachycardia. The dose of adenosine required to terminate episodes of supraventricular tachycardia was variable (range 2 to 23 mg). Side effects were minor and of short duration. These results demonstrate that adenosine is useful for the acute therapy of supraventricular tachycardia whenever reentry through the AV node is involved. When arrhythmia termination is not affected, atrial activity may be more readily analyzed during adenosine-induced transient AV block.