Phosphodiesterase III (PDE III) inhibitors are clinically used in the treatment of acute heart failure and myocardial depression. PDE III inhibitors selectively inhibit cyclic AMP, specifically PDE III in both cardiac and vascular smooth muscle, which can produce vasodilatation. Recently PDE III inhibitors are available in the treatment of cerebral vasospasm after aneurysmal subarachnoid hemorrhage. Olprinone, a newly developed PDE III inhibitor, has recently become available for the same purpose. In this study, we investigated direct free radical scavenging activity of olprinone measured by electron spin resonance with spin trapping method. ESR study has demonstrated that olprinone can scavenge hydroxyl radicals in a dose dependent manner. This result indicates that PDE III inhibitor has a direct free radical scavenging activity.