Patients having major surgery usually cannot eat or drink much for a few days. Instead, they receive an infusion of intravenous fluids. Standard practice is to give an excess of fluids, as a high urine production is thought necessary in surgical patients. Scientific studies, however, have recently shown that an excess of fluids in surgical patients approximately doubles complication rates. This study therefore challenges this tradition, by comparing the standard high urine production target (0.5 ml/kg/h) with a lower target for urine production (0.1 ml/kg/h) in 40 otherwise well patients having major bowel surgery. To ensure safety for the kidneys, novel molecules that are very sensitive to kidney injury will be measured, along with kidney blood flow and kidney function. About 45% of people in the industrialised world have abdominal surgery during their lifetime. This study aims to significantly reduce the risks of surgery.