The usual initial life-threatening effect of injury is hypovolemic shock. In the hierarchical physiologic response to hypovolemia, perfusion of peripheral tissues is sacrificed early and restored late. But the usual hemodynamic and metabolic measurements of blood pressure, urine output, and base deficit are not reliable indices of peripheral perfusion. Although the Clark electrode can quantitate tissue oxygen pressure and thereby serve as an index of perfusion, its use is compromised by several technical deficiencies. Recently, an optical method (optode) using fluorescent technology has been developed for measurement of oxygen tension in subcutaneous tissue (P sgO2). Our studies compared this device with the Clark electrode in the laboratory and tested its value in both animal and clinical studies of hypovolemic shock. The results of these several studies demonstrated that: (1) the new oximeter tracked a rapid fall or rapid rise of oxygen tension between room air (150 mm Hg) and 0 mm Hg ( a glucose oxidase/catalase solution) as well as the Clark electrode without encountering its technical problems; (2) with an acute hemorrhage to 20% of base line, the PsgO2 was found to decline rapidly in parallel with the decline of mean arterial pressure (MAP). Although the MAP rapidly returned to normal after immediate complete return of all shed blood, the PsgO2 did not reach normal levels for at least 2 hours, suggesting persistent peripheral vasoconstriction. (3) Studies in progress suggest that between 35 and 78% of trauma patients (n = 18) adequately resuscitated for hypovolemia b customary criteria have a decreased level of PsgO2 for as long as 60 hours after resuscitation for injury. If care is taken to prevent other causes of catecholamine induced vasoconstriction such as pain, fear, cold, and arterial hypoxia, these several results suggest that a certain number of injured patients are inadequately resuscitated despite the return to normal of conventional hemodynamic measurements. The serial analysis of PsgO2 may assist in managing patients and promote better understanding of the responses to injury.