This thesis examines several intra-operative physiological variables which may indicate the adequacy of global oxygenation in patients undergoing major abdominal surgery. Near infra-red spectroscopy is a non-invasive method to determine tissue oxygenation based on the varying light absorption properties of oxygenated and non-oxygenated substances, such as haemoglobin. This technique was used to assess the changes in oxygen saturation of the thenar muscle (St02) in response to a vascular occlusion test (VOT) of the arm where arteriovenous stasis is achieved using a pneumatic cuff for a pre- determined period of time. Intra-operative St02 monitoring was also undertaken during surgery. Results were obtained from 20 patients who underwent major non-laparoscopic colorectal surgery. The VOT related St02 profile was different between patients who exhibited complications, determined using a modified version of the post-operative morbidity survey (POMS), and those who did not. In addition, a lower median intraoperative St02 was associated with post-operative complications. Microdialysis, an in vivo tissue sampling technique, was used to study the metabolic changes in the lumen of the colon during colorectal surgery. A number of luminal metabolites were related to oxygen delivery index suggesting a link between large bowel metabolism and global oxygenation. Central venous saturation (Scy02) reflects the balance between oxygen delivery and demand with a number of studies supporting its use as a therapeutic haemodynamic target. This variable was measured intra-operatively and examined in relation to oxygen delivery and post-operative outcome. In the 64 patients studied, no relationship between oxygen delivery index and SCV02 was found and intraoperative Scy02 was not associated with post-operative outcome.