1 Guinea-pig and rat urinary bladders, rat stomach and the guinea-pig gall bladder, four isolated organs that show high sensitivity to bombesin, were used to characterize bombesin receptors in peripheral organs. 2 The order of potency of agonists was determined with several naturally occurring peptides of the bombesin series, namely bombesin (BBS), litorin (Lit), neuromedin B (NMB), the gastrin-releasing peptide (GRP 18-27), neuromedin C (NMC) and with some bombesin fragments. It was found that bombesin, neuromedin C, litorin and two bombesin fragments, BBS (6-14) and AcBBS (6-14) had similar activities in the four preparations, while neuromedin B and [Phe6]-neuromedin C were more active on the rat urinary bladder than on the other tissues. 3 The order of potency of agonists determined in the rat urinary bladder was as follows: BBS = NMB greater than Lit greater than NMC greater than [Phe6]NMC = GRP and it was found to be different from that observed in the other preparations: BBS greater than GRP = Lit greater than or equal to NMC much greater than NMB greater than [Phe6]NMC, suggesting the existence of two different bombesin receptors, BBS1 and BBS2. 4 This interpretation was convalidated by the finding that bombesin antagonists, namely Ac.GRP(20-26)OCH3 and Ac.GRP(20-26)OC2H5 reduced or blocked the effects of bombesin-related peptides on BBS2 receptor systems while being completely inactive on the rat urinary bladder (BBS1 system).