To monitor hypoperfusion of the peripheral tissues in critical illness caused by injury, we measured the concentrations of glucose, pyruvate, and lactate in traumatized and ischemic hamster flap tissue with the use of microdialysis. The interruption of the anatomic blood supply led to a drastic decrease in microvascular blood flow (laser Doppler flowmetry) and partial tissue oxygen tension (dye fluorescence quenching technique) in the ischemic part of the flap (both P < 0.01). In the traumatized area, blood flow, oxygen tension, and pyruvate were similar to the healthy control tissue throughout the experiments, whereas pyruvate was reduced in the ischemic tissue (P < 0.05 versus baseline and other tissues). Lactate was increased in both parts of the flap (P < 0.01 versus baseline and other groups for ischemic, not significant for traumatized). The sensitivity to detect ischemic hypoxia was 62% for lactate and 93% for lactate/pyruvate ratio (L/P) (P < 0.01). The specificity to discern ischemia-related from trauma-related changes was 71% for lactate and 70% for L/P (not significant). Our results suggest that L/P is more accurate than lactate for monitoring ischemia-related hypoxia after trauma. However, the rate of increased values originating from normally perfused but traumatized tissue was high for both markers.