Changes in total and free thyroid hormones, reverse triiodothyronine (rT3) and TSH were followed in 64 patients undergoing surgery and divided into three groups according to nature of surgery. Group I patients underwent extraperitoneal surgery, group II mild and group III heavy intraperitoneal surgery. A significant fall in total and free triiodothyronine (T3), a rise in rT3, free thyroxine (FT4) and in TSH were noted in each group after surgery, while total T4 levels remained unchanged. There was no correlation between the change in thyroid hormones and TSH. These changes are more marked in groups II and III than in group I. The rT3/T3 ratio was more elevated after surgery in group III than in group II. Thus the severity of the stress induced by surgery may interact with the mechanism of peripheral T4 conversion. The rise in FT4 may be interpreted as a consequence of the presence of an inhibitor of thyroid-hormonal binding to serum proteins. The slight increase in TSH levels seems rather due to stress than to a feed-back adaptation. The ratio rT3/T3 appears as an index of severity and prognosis.