The tissue oxygen tension (Po2) changes in and around an experimental muscular burn were directly measured. With a double labeling technique (intravenously given colloidal carbon followed by intravenously administered Evans blue) biphasic changes in vascular permeability were mapped out around a 1-mm rat cremaster burn. Direct measurements of tissue Po2 were then made with antimony-glass microcathodes mounted on a micromanipulator. The results show a significant diminution in measurable Po2 in the unstable zone of burn edema in the first 45 minutes following burn. Steep oxygen gradients exist over very short distances and the hypoxic areas correspond to the observable area of vascular stagnation and thrombus formation.