Résumé : Intraoperative hormonal measurement has been successfully used to guide the surgical treatment of various endocrine diseases. In this study, we report the results of intraoperative insulin measurement in patients with organic hypoglycemia (n = 52 operations in 51 patients) and the results of intraoperative gastrin measurement in patients with gastrinoma (n = 20). Measurements were done in the systemic and portal blood at the beginning of the operation, and 20 min after removal of the lesion(s), with intra-operative secretin stimulation test for gastrinoma in some cases. Results accurately predicted cure (insulinomas) or non-cure (half of gastrinomas) of the disease. Limitations of the method are the possibility of normal hormone base line levels at the time of surgery and the importance of secretion of pro-insulin products by insulinomas not taken in account by the assay with monoclonal antibodies.