This study was designed to study the importance of lumenal pH and antral distention on G-cell numbers. Antrocolic transpositions (ACT) were performed in female Sprague-Dawley rats, orienting the peristalsis of the transposed antra to result in either filling or emptying. In some animals a segment of parietal cell bearing corpus mucosa was left with the antrum to provide a relatively acid lumen. Sham operations were performed in controls. The rats were sacrificed after 15 days. The self-filling antra were grossly distended compared with the self emptying antra. G cells were quantitated by means of a double-antibody immunofluorescence technique. All ACT rats had significantly increased G-cell numbers and serum gastrin levels compared with controls. The magnitude of the increase depended on the presence or absence of distention and an acid environment. The most marked hyperplasia occurred in the group with distended antra and no acid. The group with self-filling antra but an acid lumen showed a significant but lesser increase. The least degree of hyperplasia was observed in the group with self-emptying antra and an acid lumen. These data indicate that both distention and a nonacid lumenal pH encourage G-cell hyperplasia. The nonacid lumen is a more potent stimulus than distention but the two are synergistic.