1. The effect of [D-Phe6] bombesin (6-13) methylester (OMe), a newly developed potent antagonist of bombesin receptors, has been investigated against bombesin-induced contractions of the guinea-pig and rat isolated urinary bladder. 2. Bombesin (0.1 nM-10 microM) produced a concentration-dependent contraction of the guinea-pig isolated bladder which approached the same maximum response as KCl (80 mM). The response to bombesin was antagonized in a competitive manner (rightward shift of the concentration-response curve without depression of the maximal response) by [D-Phe6] bombesin (6-13) OMe (0.3-10 microM). Degree of antagonism was concentration-dependent between 0.3 and 3 microM (dose ratios = 2.4, 9 and 39 in the presence of 0.3, 1, 3 microM of the antagonist). However, a larger concentration (10 microM) of the antagonist was not more effective (dose ratio = 36) than 3 microM. 3. Neither the action of bombesin nor the activity of the antagonist was influenced by peptidase inhibitors (bestatin, captopril and thiorphan 3 microM each) or by atropine, indomethacin, chlorpheniramine and desensitization of P2x purinoceptors by alpha, beta methylene ATP. 4. The bombesin antagonist was ineffective against contraction of the guinea-pig urinary bladder produced by the NK-1 tachykinin receptor-selective agonist, [Sar9] substance P sulphone. The action of the NK-1 receptor agonist was antagonized by L 668, 169 (3 microM), a cyclic peptide tachykinin antagonist. L 668, 169 had no effect toward bombesin-induced contraction. 5. The bombesin antagonist (1-10 microM) had no effect against the non-adrenergic non-cholinergic response of the guinea-pig isolated urinary bladder to electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)