Clinical measures often fail to detect early tissue ischemia in inadequately perfused flaps. This study investigates the application of a new pH electrode system to monitor tissue metabolism continuously in porcine and human musculocutaneous flaps. Bilateral rectus abdominis flaps based on the deep inferior epigastric pedicle were elevated in eight mixed-breed pigs. Tissue pH was measured in the subcutaneous and muscular layers with miniature glass-tip electrodes. The deep inferior epigastric artery and vein were serially and then concomitantly occluded for 20-minute periods. The decrease in tissue pH was greater following either arterial or pedicle occlusion (each 0.21 ± 0.03 units) when compared with venous occlusion (0.10 ± 0.02 units; p < 0.02). Changes in muscle and subcutaneous hydrogen ion concentration were similar during arterial and venous occlusions (probability not significant). Measurement of subcutaneous pH appears to be a reliable experimental and clinical physiological tool for detecting early tissue ischemia in reconstructive flaps.