The neuroendocrine responses to surgical stress in man include release of ACTH, GH and prolactin in the serum(Cooper and NElson, 1962: Ney et al., 1963: Ross et al., 1966: Schlach, 1967: Gordon et al., 1972: Jeffrey et al., 1977). Data on serum LH and FSH during operation under general anesthesia have been conflictiong. Evidence has also been accumulated that serum LH levels are increased significantly in male patients during operation under general anesthesia in comparison of serum LH in males might be different from that in postmenopausal females with a very high basal level of LH and from menstrating females. Effects of surgical stress under general anesthesia with halothane-N2O on serum LH and FSH levels were studied in 12 menstruating female patients, 8 postmenopausal female patients, and in 8 male patients with no endocrine disorders, liver or kidney function impairment as judged by routine tests. Control serum samples were taken immediately before anesthesia and subsequent samples were obtained from the patients 30 minutes, 1hour and 5~6hours after the onset of anesthesia and on the second and seventh postoperative day. The concentrations of serum LH and FSH were measured by a specific and sensitive radioimmunoassay method. The results are as follows; 1) Preanesthetic levels of serum LH and FSH served as controls and were within range of normal values for male and female subjects in our laboratory. 2) In male patients, serum LH levels 1 hour after onset of anesthesia increased significantly over those of preanesthesia, while no significant intraoperative increase in LH levels was found in female patinets. 3) No significant change in serum LH levels was demonstrated on the second and seventh postoperative day except female patients, who showed significant decrease in the serum LH level on the second postoperative day. 4) No significant intracperative and postoperative changes in serum FSH levels were observed in male or female patients. From the above results, it may be concluded that significant intraopertive increase in serum LH levels occurs in male patients but not in female patients.