We hypothesized that metabolites of O2 may play a role in the development of airway hyperreactivity and undertook this study to examine the effects of one of these metabolites, hydrogen peroxide (H2O2), on electrical field stimulation-induced contractile responses of isolated rat intrapulmonary bronchi. Exposure to H2O2 (1 mM) elicited a transient contractile response with a peak response equivalent to 18.1 +/- 2.0% of the reference contraction obtained to electrical stimulation. The H2O2-induced contraction was attenuated by pretreatment of tissues with indomethacin and superoxide dismutase, but abolished by catalase and mianserin. Subsequent to H2O2 exposure, electrical field stimulation-induced contractile responses were potentiated (P less than 0.0001), whereas acetylcholine-induced contractions were not. The potentiating effects of H2O2 were inhibited by catalase and mianserin. Addition of 5-hydroxytryptamine (5-HT) to the bath similarly potentiated contractions to electrical stimulation (P less than 0.0001). Together, these results are consistent with a role for 5-HT in H2O2-induced contraction and the subsequent potentiation of airway smooth muscle contraction elicited by cholinergic nerve activation. Thus endogenous metabolites of O2 may be important in modulating airway smooth muscle tone.