On the basis of the positive results of recent experimental research, a clinical trial of phosphocreatine (Neoton) was carried out in 60 randomized patients with acute myocardial infarction (30 patients in the Neoton group and 30 patients in the control group). Neoton was given intravenously not later than 6 hours after the onset of symptoms, in a dose of 2 gm as a bolus injection, followed by a 2-hour infusion at the rate of 4 gm/hr. Holter monitoring for 24 hours showed a significant decrease in the frequency of ventricular premature beats: in the Neoton-treated group the total number of ventricular premature beats for 24 hours was 690 +/- 179 vs 2468 +/- 737 in the control group (p less than 0.02). During this period of time the number of ventricular tachycardia paroxysms was 6 +/- 2 in the treated group and 97 +/- 35 in the control group (p less than 0.01). No side effects or complications were found after administration of phosphocreatine. It is concluded that phosphocreatine may be a potentially important antiarrhythmic drug for treatment of patients with acute myocardial infarction.