Serum Ta and T were measured in 34 patients scheduled elective surgery with general anesthesia, who were divided into two groups (enflurane group' 12, propofol group' 22). No clinical signs of thyroid disturbances could be observed preoperatively in the patients. Anesthesia was induced with thiopental 5 mg/kg in enflurane group; propofol 2 mg/kg in propofol group, and maintained with Oz 2 L/min, NH 2 L/min, enflurane (1.0-2.596) in enflurane group,' 2 L/ min, N 4 L/min and continuous controlled infusion of pmpofol (50-150 mcg/kg/min) in propofol group. Blood samples were collected 5 minutes after arrival in operating room, 1 minute after induction, 5 minutes after intubation, 1 minute after skin incision, during operation and 10 minutes after extubation. Blood samples were centrifused and serum was taken from each sample. Determinations of Ta and T4 were performed using enhanced chemiluminescence immunoassay (ELIA). Comparisons of our results were made using repeated measuves analysis of variance, paired students t-test and statistically significant when p value was less than 0.05. The results are summarized as follows' 1) In enflurane group, serum Ta levels showed a decreasing tendency after extubation compared to baseline. Decreasing serum T4 after administration of thiopental could be considered statistically significant and serum T levels showed a decreasing tendency after extubation compared to baseline. 2) In propofol gnup, serum Tz levels showed a decreasing tendency after extubation compared to baseline. Rises in serum Te after incision and extubation could be considered statistically significant. Based on these results, no direct negative influence of propofol need be expected on Tg metabolism in euthyroid patients. However, the changes in these hormones, although with in the physiological range here, indicate that propofol may adversely influence their levels in the presence of manifested thyroid disease.