Biorhythms, such as regular variation in core body temperature and the pattern of the secretion of melatonin, are thought to be mediated by the same biological clock. Core body temperature is affected by the inflammatory response to major surgery. Apart from the well-known inhibitory effect of bright light on its secretion, melatonin is an exceedingly good marker of one of the central generating systems of circadian rhythms. We sequentially measured the plasma melatonin concentration pattern in patients who had undergone esophagectomy with thoracotomy to elucidate the circadian rhythm after major surgery. From seven patients who had received esophagectomy with thoracotomy for esophageal cancer, plasma concentrations of melatonin were measured using an RIA method. Blood samples were collected via each patient's arterial line at 00.00, 02.00, 04.00, 06.00, 08.00, 12.00, 16.00, 20.00, and 24.00 hr on the first postoperative day for six of the patients, and, for one patient, every 2 hr until the third postoperative day and every 4 hr thereafter until the sixth postoperative day. Four patients out of seven had melatonin concentrations of over 30 pg/ml (mean 34 pg/ml) at 24.00 hr on the first postoperative day. Five patients showed circadian secretion patterns of melatonin during the first postoperative day. One patient whose melatonin concentrations were measured consecutively for 6 days showed a regular circadian secretion pattern through the 6 days of the study. Even the stress caused by extremely invasive surgery did not significantly disturb the melatonin secretion pattern.