Bradykinin (BK) caused the circular muscle of opossum lower esophageal sphincter to relax and then contract in vitro. The effects of BK were not modified by indomethacin, tetrodotoxin, omega-conotoxin, atropine, propranolol, phentolamine, haloperidol, methysergide, pyrilamine or cimetidine. Apamin but not tetraethylammonium antagonized the inhibitory effect of BK and nifedipine antagonized its excitatory effect. Structure-activity relationships of 10 BK analogs known to be active on BK receptors were studied. Six analogs had distinct excitatory response profile and the rank order of potency of these agonists according to pD2 (shown in parentheses) was D-Arg-[Hyp3Thi5,8D-Phe7]-BK (6.49 +/- 0.19) greater than [Thi5,8D-Phe7]-BK (5.61 +/- 0.26) greater than Lys-Lys-[Hyp3-Thi5,8D-Phe7]-BK (5.45 +/- 0.14) greater than BK (5.16 +/- 0.20) greater than [des-Arg9]-BK (4.95 +/- 0.08) greater than [D-Phe7]-BK (4.73 +/- 0.10). However, only BK was fully efficacious (100%) and Emax values of all the analogs were less than that of BK. On the inhibitory response BK was the only agonist (pD2 = 5.48 +/- 0.08) and none of the BK analogs were agonist. All the BK analogs were also inactive as antagonists except Lys-Lys-[Hyp3-Thi5,8D-Phe7]-BK and Lys-Lys-[Hyp2,3-Thi5,8D-Phe7]-BK which competitively and selectively antagonized the BK inhibitory response. The pA2 value of Lys-Lys-[Hyp3-Thi5,8D-Phe7]-BK was 6.92 +/- 0.17.(ABSTRACT TRUNCATED AT 250 WORDS)