Acute arterial obstruction to the extremities is associated with significant morbidity and mortality. The evaluation of accompanying skeletal muscle injury has thus far been indirect and imprecise. Triphenyltetrazolium chloride (TTC) is an oxidation-reduction indicator which allows for the histochemical quantitation of skeletal muscle injury. In 21 anesthetized nonheparinized adult mongrel dogs, the isolated in vivo gracilis muscle underwent 4, 6, or 8 hr of ischemia with and without reperfusion. The muscles were excised and cut into 1-cm segments, representative muscle biopsies for electron microscopy were taken, each segment was stained in 1% TTC, and the total area of staining was measured with computerized planimetry. All control muscles stained completely with a dark red color. After 4, 6, or 8 hr of ischemia, quantitative measurements of muscle staining indicative of normal tissue were present in 98 ± 1%, 59 ± 5%, and 23 ± 9% of the total muscle areas, respectively. Six hours of ischemia followed by reperfusion was associated with only 36 ± 9% of the muscle being stained. Segmental TTC staining demonstrated that reperfusion was associated with greater injury, and less TTC staining, in the proximal portion of the gracilis muscle at the site of entry of the major arterial pedicle. The distal muscle did not demonstrate increased damage with reperfusion. It is hypothesized that protection of the distal muscle from reperfusion injury may be due to an absence of reflow farther away from the artery.