Several families of very potent bombesin (Bn) receptor antagonist analogues have recently been developed and their biological potencies evaluated in a number of in vitro systems including guinea pig and rat pancreatic acini and Swiss 3T3 cells. These studies showed that analogues can exhibit diverse properties ranging from full antagonists, partial agonists, or full agonists depending on the assay system and animal species employed. We have developed two classes of more potent, shorter chain antagonists based on [psi CH2NH(13-14)]Bn(6-14) and desMet14Bn(6-13)NH2 structures. [D-Phe6 psi Leu13-Leu14] Bn(6-14)NH2 was a potent antagonist (Ki 6nM) in Swiss 3T3 cells and guinea pig acini but exhibited 10% partial agonist activity and lower binding affinity (Ki 60 nM) in rat acini. The partial agonism could be eliminated by using p-Cl-Phe or D-Phe at the C-terminus and partially eliminated using D-4-Cl-Phe in position 6. With the antagonist [D-Phe6]Bn(6-13)NH2 (Ki 96 nM), alkyl substituents on the amide group increased affinity 25-fold with the propylamide being the most potent peptide (Ki 4 nM) in 3T3 cells or guinea pig acini. It did, however, have high 40% partial agonist activity in rat acini. Alkyl esters or hydrazide derivatives were, in contrast, pure antagonists in all systems tested with [D-Phe6]Bn(6-13)OMe having the highest affinity in all systems and also excellent in vivo properties. All of the potent antagonists examined had little affinity for neuromedin B--preferring bombesin receptors, which had entirely new ligand structure-activity relationships.(ABSTRACT TRUNCATED AT 250 WORDS)