Intravenous adenosine proved effective in eight out of ten episodes of acute spontaneous supraventricular tachycardia. AV node blockade induced by adenosine occurred 20 s after injection, and disappeared very quickly. Side-effects were transient and never serious. Therefore, it is thought that adenosine can be proposed safely for the short-term treatment of supraventricular tachycardia. Because of the different mechanisms of action, the specific side-effects observed after ATP administration are unlikely to be encountered.