We investigated postoperative changes in immunoreactive plasma endothelin (ET-1) concentrations using a highly specific and sensitive radioimmunoassay developed for this purpose. The patients were divided into two groups: 10 patients underwent subtotal-esophagectomy by combined right thoracotomy and celiotomy (EC-group), and an other 10 underwent total gastrectomy via celiotomy only (GC-group). Before operation, plasma ET concentrations in both groups were within the normal range. After operation, the mean plasma ET-1 concentration in the EC group was significantly higher than the preoperative concentration (p less than 0.01). But in the GC-group, there were no significant change. A significant correlation existed between the change in plasma ET-1 concentration and the amount of intraoperative hemorrhage (p less than 0.05). Additionally in the EC group, a significant correlation existed between plasma ET-1 concentrations after operation and the change in the atrial-natriuretic-peptide (ANP) concentration during operation (p less than 0.05). There was no correlation between ET-1 concentrations and blood pressure. We believe that the increase in the ET-1 concentration was attributable to endothelial cell injury and may increase ANP secretion. We speculate that the plasma ET-1 concentration may be a reliable index of surgical stress.