The viability of tissue flaps depends on adequate blood flow and oxygenation. To help ensure oxygen delivery, increased inspired oxygen is often provided. This study uses a porcine model to measure tissue oxygen (TPO2) in a muscle flap, in response to varying levels of inspired oxygen concentration (FiO2). Six swine underwent the creation of a latissimus dorsi island flap. An ultrasonic flow probe was used to monitor afferent flow through the thoracodorsal artery, and a 20-ga fluorescence-quenching optode was employed to monitor TPO2. Additional optodes were inserted in muscle of an ipsilateral hindlimb, and in the terminal ileum. Inspired oxygen concentration was varied from 15 to 100%, and oxygen delivery variables measured. Analysis of variance and multiple linear regression were used to determine which variables had the greatest effect on TPO2. All three sites varied directly with inspired oxygen concentration. Flap TPO2 had a strong dependence on FiO2 and local oxygen delivery (r2 = 0.54). PaO2 and hemoglobin were the most significant determinants of ileal submucosal TPO2 (r2 = 0.65). A correlation between average submucosal and flap TPO2 was observed (r > 0.9, P < 0.05). We conclude that (1) muscle flap and bowel TPO2 vary directly with inspired FiO2, (2) changes in ileal submucosal TPO2 correlate with those observed in muscle, and (3) monitoring of readily accessible muscle TPO2 merits further investigation to evaluate the status of TPO2 in critical visceral beds.