The presence of vasoactive intestinal peptide in intramural neurons of the stomach and in nerve fibers of antral and fundic mucosa raises the possibility that vasoactive intestinal peptide may participate in the regulation of gastrin secretion or somatostatin secretion, or both. This possibility was examined in the isolated vascularly perfused rat stomach by measuring the effects of vasoactive intestinal peptide and its homolog, secretin, alone or in combination with somatostatin antiserum on gastrin and somatostatin secretion. Both vasoactive intestinal peptide and secretin caused a sustained increase in somatostatin secretion in accordance with previous reports by others, but only secretin caused a sustained decrease in gastrin secretion; vasoactive intestinal peptide caused a transient increase in gastrin secretion followed by a return to control levels. Infusion of somatostatin antiserum in combination with vasoactive intestinal peptide caused a significant and sustained increase in gastrin secretion; however, the increase was not greater than that previously found with infusion of somatostatin antiserum alone. Infusion of somatostatin antiserum in combination with secretin caused a transient (2 min) increase in gastrin secretion followed by a sustained decrease in gastrin secretion equal to that caused by secretin alone. The results suggest that vasoactive intestinal peptide participates in the regulation of somatostatin but not of gastrin secretion. The results also suggest that basal secretion of somatostatin exerts an optimal inhibitory effect on gastrin secretion that is not exceeded by further increase in somatostatin secretion. At high concentrations, secretin has a direct inhibitory effect on gastrin secretion but this effect is not likely to be physiologically relevant.