The purpose of this project was to assess whether a novel, small diameter two-electrode oxygen sensor is capable of early detection of hemorrhage-induced hypoperfusion in rats and pigs. Hemorrhage is a very common cause of death after trauma and the degree of blood loss is difficult to gauge. In order to monitor blood volume, we developed a sensor designed to rapidly respond to changes in subcutaneous oxygen tension. The oxygen sensor included a platinum indicating electrode and a AgCl-based reference electrode. When tested in rats and pigs during controlled hemorrhage, the signals from these subcutaneously implanted sensors closely tracked the mean blood pressure and venous pO2. The sensor signals declined quickly during blood withdrawal and rose quickly in response to fluid resuscitation. Pulse rate was not a reliable indicator of blood loss. The signals of the oxygen sensors declined (negative slope) during hemorrhage and rose (positive slope) during resuscitation. In all cases, in both rats and pigs, slopes of less than -0.1 Torr/min indicated hemorrhage and slopes of more than 0.1 indicated resuscitation. Given the similarities of skin and subcutaneous tissue in pigs versus humans, devices like this may be useful for early detection of hemorrhage in humans.