We studied the effect of in vivo ozone exposure (3 ppm, 2 h) on methacholine- and histamine-induced guinea pig tracheal smooth muscle contractions in vitro and the role of cyclooxygenase products in this process. After exposure to ozone, methacholine stimulation showed a functional hyperreactivity, whereas after stimulation with histamine a hyporeactivity was observed. These effects could be explained by the release of prostanoids. In a control situation an increase in PGF2, PGE2 and PGD2 release is observed after stimulation of the histaminergic receptor system. After ozone exposure the release of prostanoids was also enhanced (unstimulated, PGF2 and TxB2; histamine, PGF2, PGE2; methacholine, PGF2, TxB2, 6-kPGF1, PGE2). This study shows that the prostanoid release is strongly dependent on the receptor system stimulated to induce smooth muscle contraction and the importance of prostanoids in ozone-induced changes in guinea pig tracheal smooth muscle reactivity.