Thirteen proglumide derivatives that varied in the length of the di-n-alkyl group and in the substitutions on the benzoyl moiety were tested for their ability to interact with guinea pig pancreatic cholecystokinin (CCK) receptors. Each derivative was more potent than proglumide. There was a close correlation between their abilities to inhibit CCK-stimulated amylase release and to inhibit binding of 125I-CCK. For the di-n-alkyl derivatives the relative potency was n-pentyl greater than n-hexyl greater than n-butyl greater than n-propyl. For the benzoyl moiety, adding two electron-withdrawing groups increased potency more than adding a single electron-withdrawing group or adding electron-donating groups. The 3,4-dichloro-di-n-pentyl derivative of proglumide was 1,300 times more potent than proglumide, and its action was specific, competitive, and it functioned as a CCK receptor antagonist in rat, mouse, and guinea pig pancreas. For all proglumide derivatives there was a good correlation (r = 0.84, P less than 0.001) between their abilities to inhibit CCK-stimulated amylase release and that previously reported for their abilities to inhibit CCK-induced gallbladder contraction. However, certain proglumide derivatives had a much higher affinity for the pancreatic CCK receptor than for the CCK receptor mediating gallbladder contraction. For other proglumide derivatives the pattern was reversed. These results demonstrate that both the di-n-alkyl group and the substitution on the benzoyl moiety of proglumide are equally important determinants of affinity and that derivatives such as the di-n-pentyl 3,4-dichloro analogue can be produced that are 1,300 times more potent than proglumide.(ABSTRACT TRUNCATED AT 250 WORDS)