Rectangular skin flaps based on the right superficial epigastric vessels were designed on the groins of 36 rats. Preoperative control, intraoperative, and postoperative readings of oxygen tension (PO2) were made at proximal, central, and distal sites on the flaps with a transcutaneous PO2 (tcPO2) monitor under various conditions of oxygen inspiration. The results of this experimental work indicated that the tcPO2 monitor was useful in continuously and rapidly measuring changes in oxygen concentration in skin flaps in a noninvasive fashion. The monitoring demonstrated that the response time of the flaps to changes in the concentration of inspired oxygen was rapid (less than 15 seconds). The monitoring also was valuable in assessing viability of the flaps, in predicting flap survival, and in detecting any systemic factors influencing oxygen transport, such as pneumonia. As a result of the experimental series, tcPO2 monitoring was used clinically to evaluate 18 flaps in 16 patients. As in the experimental series, the clinical measurements were significant and reproducible. They demonstrated that the tcPO2 monitor provides safe, reliable monitoring of peripheral oxygenation in the microcirculation that is rapid, continuous, and totally noninvasive. It is concluded that simultaneous tcPO2 measurements at control and flap sites provides a continuous record of the status of a flap that can improve the postoperative management of the surgical patient.