A study was carried out to determine the time course and degree of postoperative insulin resistance in patients undergoing elective abdominal surgery. Mean(s.e.m.) insulin sensitivity was determined before and on the first (n = 10), fifth, ninth and 20th (n = 5) days after elective open cholecystectomy using the normoglycaemic (4·7(0·1) mmol/1), hyperinsulinaemic (402(12) pmol/1) glucose clamp technique. Preoperative insulin sensitivity expressed as the M value varied from 2·3 to 8·2 mg per kg per min. The relative reduction in insulin sensitivity was most pronounced on the first day after surgery, at a mean(s.e.m.) of 54(2) per cent. Thereafter, a large variation between individuals was found during the course of recovery, and insulin sensitivity returned to normal 20 days after operation. On the first day after surgery, plasma concentrations of glucose, C peptide, noradrenaline and glucagon were slightly but significantly higher than before operation (P < 0·05), whereas insulin, growth hormone, cortisol and adrenaline levels were unaltered. Marked insulin resistance thus develops after elective upper abdominal surgery and persists for at least 5 days after operation. Factors other than simultaneous changes in levels of the hormones studied seem to regulate the maintenance of postoperative insulin resistance.