Gastric bypass, a 90% gastric exclusion, has been used successfully for surgical treatment of morbid obesity since 1967. Early concern as to its ulcerogenic potential has not materialized, but the physiological activity of the excluded stomach has not been studied fully. To determine whether the excluded segment retained any vagal innervation, 25 patients underwent preoperative Hollander tests of their intact stomachs. The test was repeated after operation, after the patients had resumed normal oral intake. Postoperative specimens were collected from a gastrostomy placed in the excluded stomach at the time of the gastric bypass. Percentage changes in volume and total acid for both the intact and excluded stomachs followed the same pattern with insulin injection after 45 minutes at both testings, pH values were identical. These observations, which indicate vagal innervation of the excluded stomach, together with previously published histalog acid-response data and the observed marginal ulcer incidence of one per 193 years of patient follow-up, demonstrate that normal gastric physiology is maintained after bypass and confirm that it is not an ulcerogenic procedure.