Plasma testosterone levels were determined by the competitive protein binding method of Mayes & Nugent (1968) with slight modifications, in 20 male patients in whom moderate or major surgery had been performed. Preoperative (control) and postoperative blood samples were obtained at 8 a. m. except for immediate postoperative samples. There was a significant decrease in the plasma levels of testosterone immediately and 2 days after moderate and major surgery. The level of testosterone returned toward the control level on the sixth postoperative day after moderate surgery, while after major surgery, the decreased plasma testosterone was maintained for at least 6 days. It is suggested that testicular secretion of testosterone seems to decrease during the postoperative period.