To document that the previously demonstrated excitatory effects of cholecystokinin-pancreozymin--about 20% purified and containing approximately 500 U/mg (CCK-PZ 20%)--on isolated guinea-pig antrum and fundus are caused by the hormone itself, and not by impurities, the effect of the synthetic C-terminal octapeptide (OP-CCK) was tested and compared with that of CCK-PZ 20%. The qualitative effects were similar for the two hormones, characterized by increased amplitudes of rhythmic activity in the antrum and by elevated basal tension in the fundus. The calculated maximal responses were about the same for the two substances, but OP-CCK was 3--6 times more potent than CCK-PZ 20%. Atropine and tetrodotoxin markedly reduced the antral response to OP-CCK, while the fundal response was only slightly, but significantly, reduced. This indicates that the antral response to OP-CCK is mediated mainly through local neural cholinergic pathways, while the fundal response is less dependent on these, corresponding to our previous results for CCK-PZ 20%. While previous reports indicate that OP-CCK shares the pharmacological characteristics of partially purified CCK-PZ with regard to its effect on other organs, the present study confirms a corresponding relationship between OP-CCK and CCK-PZ 20% on gastric motility in guinea-pigs, suggesting that the actions of CCK-PZ 20% are displayed by the hormone itself, and not by impurities.