We tested the hypothesis that selected hormonal responses to surgery reflect the degree of surgical stress. Plasma norepinephrine, epinephrine, thromboxane B2, cortisol, serum angiotensin converting enzyme, thyroxine, triiodothyronine, free thyroxine, and free triiodothyronine levels were measured preoperatively, and then one hour, 24 hours, and five days postoperatively in three groups of patients. The groups were as follows: group 1, "minimal" stress, eg, inguinal hernia repair (n = 10); group 2, "moderate" stress, eg, cholecystectomy (n = 12); and group 3, "severe" stress, eg, subtotal colectomy (n = 9). Patients in group 1 showed no significant surgery-induced changes in hormonal values. The stress-induced changes in patients in groups 2 and 3 were seen at one and occasionally 24 hours; however, by five days postoperatively, circulating hormone values had returned to preoperative levels. Increases in plasma cortisol, norepinephrine, and epinephrine, and decreases in serum angiotensin converting enzyme levels characterized the surgery-induced hormonal changes. Conclusions are as follows: hormonal responses do reflect the degree of surgical stress; the hormonal changes are transient, lasting no longer than 24 hours in patients after uncomplicated surgery; hormonal responses to minimal surgical stress are negligible.