An artificial beta cell has been used to achieve and maintain a preset plasma glucose concentration in five diabetic patients undergoing surgery. These subjects were compared to control groups of normal subjects receiving either saline or glucose, and diabetics receiving glucose intraoperatively. Hyperglycaemia during surgery was seen in normals (mean plasma glucose +/- SEM: 185 +/- 16 mg/dl) and, to a greater degree, diabetics (247 +/- 36 mg/dl) receiving glucose. Insulin and C-peptide levels did not increase during 2 hours of operation in any of the control groups, suggesting beta cell suppression during surgery. As C-peptide levels declined similarly in normal subjects whether they received saline or glucose, the hyperglycaemia seems to be due to an inability to use exogenous glucose. This is confirmed by a correlation of maximal plasma glucose to glucose infusion rate (r = 0.78, p less than 0.01). The artificial beta cell was able to achieve the same plasma glucose after 2 hours of operation (128 +/- 21 mg/dl) as normal subjects receiving saline (110 +/- 7 mg/dl). The artificial beta cell proved to be a safe, convenient and effective way of monitoring and controlling the hyperglycaemia seen in diabetic patients undergoing surgery.