This study tests the possibility that the difference in acid reduction after parietal cell vagotomy (PCV) and selective gastric vagotomy with a drainage (SGV + D) could be explained by differences in the secretion recovered rather than by real secretory differences. A method for estimating the completeness of aspiration during gastric function tests using the isotope Nai251CrO4 as an inert marker is described. The method was used in patients with the two types of vagotomy and the recovery of gastric secretion was more complete after PCV (89.7%) than after SGV + D (75.1%). This difference may substantially account for the apparent difference in reduction of acid secretion after the two operations. The pyloroplasty supplementing the SGV is probably responsible for a pyloric loss causing a lower recovery after this operation.