Pre- and peroperative determination of glomerular filtration rate (GFR) was performed in nine patients operated during light halothane anaesthesia (group A) and in nine patients operated during epidural analgesia in combination with light halothane anaesthesia (group B). In group A, the mean GFR decreased insignificantly by 7% and the mean arterial blood pressure increased significantly by 10%. In group B, the mean GFR decreased significantly by 19% and the mean arterial blood pressure decreased significantly by 15%. It is suggested that the difference in change in GFR between the two groups was caused by differences in mean arterial blood pressure.