The usefulness of acid secretory data collected during a modified sham-feeding (MSF) test was examined in 50 patients suffering from duodenal ulcer before and after parietal cell vagotomy. In these patients basal acid output pentagastrin-stimulated acid output and MSF-stimulated acid output were measured. Preoperative MSF-stimulated acid output was found to be correlated with preoperative basal acid output, and postoperative MSF-stimulated acid output was correlated with both preoperative basal acid output and postoperative basal and pentagastrin-stimulated acid outputs. The addition of preoperative MSF-stimulated acid output to preoperative pentagastrin-stimulated acid output did not noticeably increase the close relationship between the latter and postoperative pentagastrin-stimulated acid output. It is concluded that basal and MSF-stimulated secretory rates overlap. Therefore, it is suggested that in both preoperative and postoperative conditions the addition of the sham-feeding test to the more simple basal and pentagastrin-stimulated studies could be discarded.