Changes of plasma cortisol, prolactin and thyroxine levels as a result of surgical stress were examined in 11 patients undergoing abdominal hysterectomy. General anaesthesia was induced with the new benzodiazepine derivative midazolam (0.25 +/- 0.04 mg/kg) and maintained by nitrous oxide in oxygen. Plasma cortisol and thyroxine levels slightly decreased at the induction of anaesthesia and the beginning of surgery, while the prolactin level increased. By the end of surgery the cortisol and prolactin concentrations increased significantly, although the thyroxine level had barely changed. The highest hormone values were observed a few hours after the operation, but on the next morning hormone levels approached the normal range. On the basis of these results anaesthesia induction with midazolam can be said to have favourably affected the unwanted degree of hormonal changes related to surgery.