Intravenous adenosine triphosphate has been proved to be useful for the treatment of supraventricular tachyarrhythmias. The optimal dose to be employed, however, has yet to be stated. Forty-two episodes of spontaneous paroxysmal supraventricular tachycardia, observed in 33 patients (16 men and 17 women, mean age 51 years) were treated with intravenous boluses. These were given in 10 mg increments at increasing doses from 10 to 50 mg at intervals of three minutes if the arrhythmia persisted despite the previous dose. When using a dose less than or equal to 40 mg, tachycardia was terminated in 37 of 42 episodes in less than 50 seconds. In four of the remaining five, atrial flutter and ectopic atrial tachycardia were diagnosed after administration of adenosine triphosphate. The other patient was subsequently found to have an atrioventricular reentrant tachycardia incorporating two accessory pathways. More than half of the treated episodes of supraventricular tachycardia terminated with the minimal dose of 10 mg and 7.1% required 40 mg. In five patients, arrhythmia ceased and reappeared despite the use of adenosine triphosphate. All the patients experienced an unpleasant feeling of dyspnoea or suffocation after injection of the drug. Sinus pauses and bradycardia following termination of the arrhythmia were directly correlated with the dose employed (P less than 0.05). We conclude that adenosine triphosphate is a useful, safe and effective drug at low dose in correctly diagnosed reentrant tachycardias involving the atrioventricular node. It is also useful as a diagnostic tool in patients with supraventricular tachyarrhythmias.