This study aims to investigate the effects of ulinastatin, a human urinary trypsin inhibitor, on the cutaneous microcirculation during inhalation of 100% oxygen in the rabbit. Twenty-one rabbits having a rabbit ear chamber (REC), were divided into 2 groups: 11 rabbits without ulinastain treatment (Group C) and 10 rabbits with ulinastatin (Group U). After air inhalation, the inspired gas was changed to 100% oxygen. In Group U, 50,000 units of ulinastatin were administered before changing the concentration of inspired gas. The results suggests that ulinastatin may be useful for maintaining the cutaneous microcirculation during inhalation of 100% oxygen.