The protective effect of human urinary trypsin inhibitor(LTI) on acute hemorrhagic shock in beagle dog was studied. Hemorrhagic shock was experimentally induced in thoracotomized beagle dogs by removing blood and maintaining low arterial blood pressure for 30 min, and then blood removed was entirely transfused back into the dogs within one hour. When the blood was transfused, UTI was administered together to check the potential protective effect of UTI on hemorrhagic shock. The arterial blood pressure recovery was accelerated slightly by UTI treatment. Blood pH and P_aco₂ returned to normal level in shorter time in the UTI treatment group. These data suggest that UTI may have protective effects on experimentally induced hemorrhagic shock.