The relationship between tissue oxygen tension (PO2) and blood flow, blood gas tensions, oxygen delivery, and oxygen consumption was investigated in myocutaneous island flaps in 24 pigs. The tissue PO2 was measured by the silicone rubber tonometer technique. Capillary blood flow was increased during the first 3-4 hours after implantation of tonometers and they must, therefore, be implanted 3-4 hours before use. After raising the flap, mean (SEM) total flap blood flow increased from 11.2 (0.8) ml/min to 16.0 (1.3) ml/min within the first three hours. Mean (SEM) subcutaneous tissue PO2 decreased from 9.5 (0.6) kPa to 5.2 (0.7) kPa over six hours after the procedure, whereas muscle PO2, blood oxygen tension, and arteriovenous oxygen difference were unchanged. Hence, both the supply to, and use of oxygen in, the flap increased. Muscular and subcutaneous oxygen tensions were about 3 and 1 kPa, respectively, lower than venous oxygen tension. Tissue PO2 did not correlate with blood flow, blood oxygen tensions, arteriovenous differences in oxygen tension, or with oxygen supply in a cross section of animals. Tissue PO2 seems to reflect accurately the balance between oxygen supply and consumption, which might be the most useful indicator for continuous flap monitoring.