We determined the relative molar potencies of structural analogues of porcine cholecystokinin (CCK-39, CCK-33, CCK-8, and caerulein). Peptide concentrations delivered in infusates or present in bathing medium were measured by radioimmunoassay. The presence of albumin prevented loss of CCK-39 and CCK-33 from solution to a greater degree than loss of CCK-8 and caerulein from solution. As much as 10-fold differences in CCK-33 and CCK-39 concentrations were seen in albumin-containing versus nonalbumin-containing infusates. The potency estimates calculated from radioimmunoassay-corrected concentrations with CCK-8 as standard (potency 1.00) were canine pancreatic secretion in vivo: CCK-39 4.1, CCK-33 2.2, and caerulein 2.1; rat pancreatic secretion in vivo: CCK-39 2.1, CCK-33 5.4, and caerulein 5.4; rat pancreatic secretion in vitro: CCK-33 1.7, and caerulein 1.2; guinea pig gallbladder contraction in vivo: CCK-33 1.3, and caerulein 0.9; and guinea pig gallbladder contraction in vitro: CCK-33 1.8, and caerulein 5.8. Our data indicate that CCK-8 is not more potent than longer analogues and suggest that larger forms of CCK may be important mediators of the biological actions of CCK.