Serotonin (10(-6) - 10(-4) M) produced relaxations in a concentration-dependent (at 10(-6) and 10(-5) M concentrations) manner followed by a contraction (at 10(-4) M concentration) in a co-axial system, which consisted of guinea-pig trachea as a donor organ for epithelial derived-relaxing factor(s) and phenylephrine-precontracted rat anococcygeus muscle as assay tissue. Serotonin produced a concentration-dependent contraction only in precontracted rat anococcygeus muscle mounted alone or mounted co-axially within epithelium-denuded trachea. Indomethacin (10(-6) M) significantly inhibited the initial relaxations (from 25.1 +/- 7.8 to 7.8 +/- 5.0% and from 35.6 +/- 8.7 to 10.4 +/- 8.3% at 10(-6) and 10(-5) M concentrations of serotonin), but did not affect the contraction. Imipramine (10(-8) M) and hydrocortisone (3 x 10(-5) M) reduced the initial relaxations (from 20.5 +/- 1.6 to 3.8 +/- 1.5% and from 32.1 +/- 6.4 to 18.9 +/- 3.9% at 10(-6) M and 10(-5) M concentrations of serotonin, respectively) and also converted the serotonin (10(-4) M)-induced contraction to a relaxation. In the co-axial system with trachea from guinea-pigs previously sensitized with i.p. injected egg-ovalbumin, the serotonin-induced biphasic response was converted to a contractile response only after ovalbumin challenge. Histopathologic changes were observed in the epithelium of challenged tracheas taken from sensitized guinea-pigs and alterations of serotonin-induced epithelium-dependent responses were attributed to the morphological and/or functional damage of tracheal epithelium caused by ovalbumin challenge.(ABSTRACT TRUNCATED AT 250 WORDS)