We studied the role of endogenous prostaglandins in modulating the histamine response of canine tracheal smooth muscle (TSM) in vitro. Indomethacin (INDO) (10(-7) - 10(-5) M), a cyclooxygenase and prostaglandin synthesis inhibitor, significantly increased maximum histamine-induced tension (Tmax) and decreased the concentration of histamine required to produce 50% of Tmax (EC50). Acetylsalicylic acid (10(-5) -5 X 10(-4) M), another less potent cyclooxygenase inhibitor, also decreased EC50. Neither the lipoxygenase inhibitor nordihydroguaiaretic acid nor the leukotriene antagonist FPL 55712 had any effect on histamine-induced tension in INDO-pretreated TSM. INDO reduced the standard deviation of EC50 from 0.47 in control TSM (n = 51) to 0.26 in INDO-pretreated TSM (n = 31) (P less than 0.02). High-pressure liquid chromatography established prostacyclin (PGI2), through its degradation product 6-oxo-PGF1 alpha, as the predominant prostaglandin produced by canine TSM. Exogenous PGI2 caused a concentration-dependent relaxation of histamine-contracted TSM. In the tissue bath, spontaneous efflux of 6-oxo-PGF1 alpha from TSM, as measured by radioimmunoassay, averaged 4.7 ng . g muscle-1 . min-1 and increased to 10 ng/g muscle (n = 10, P less than 0.001) with administration of histamine. The isometric tension produced by histamine (10(-4) M) was inversely linearly correlated with the log concentration of endogenous 6-oxo-PGF1 alpha (r = 0.81, P less than 0.01). Our results are consistent with an important role for endogenous bronchodilating prostaglandins, probably prostacyclin, in determining both the histamine sensitivity of canine TSM in vitro and its variability among individual animals.