The aspartic acid residue at the penultimate position is known to be essential for the hormonal activity of CCK and gastrin on gastric acid secretion. This residue was successively replaced by beta-aspartic acid, beta-alanine, and glutamic acid in the C-terminal heptapeptide of CCK 27-33. The analogues obtained were tested on rat gastric acid secretion and for recognition by gastrin receptors. The replacement by beta-aspartic or beta-alanine decreased gastric secretion and gastrin receptor recognition. In contrast, replacement by glutamic acid affected these two parameters less. The nature of the N-blocking group (Boc or Z) also influenced these activities, Boc derivatives being more potent than Z derivatives. The results were compared to those previously obtained on pancreatic secretion and on stimulation of gall bladder contraction where the modifications were found capable of differentiating between cholecystokinin, pancreozymin and gastrin activities.