The study was carried out to investigate the variations in blood sugar values during routine fluid therapy in surgical patients, when the rate of infusion often is very haphazardly adjusted. Surgical patients with normal sugar and fluid balance were divided into two groups. The glucose group (34 patients) was given 5% glucose solution as infusion fluid and the control group (the saline group, 26 patients) received 0.9% physiological saline solution. The preoperative mean value of blood sugar in the glucose group was 4.9 mmol/1 and in the saline group 4.6 mmol/1. Half an hour after the beginning of the operation the blood sugar in the glucose group rose to 7.2 mmol/1 (p smaller than 0.001) and in the saline group to 5.3 mmol/1 (p smaller than 0.01). At the end of the operation the blood sugar values in both groups had further increased significantly from the half-hour levels, in the glucose group to 7.8 mmol/1 and in the saline group to 6.1 mmol/1 (p smaller than 0.01). The difference between the groups, at each test after the initial test, was highly significant throughout (p smaller than 0.001). The result indicates to carefulness in using glucose solutions even during operations lasting 1-2 hours.