We examined the possible role of cyclooxygenase (COX) in charybdotoxin (ChTX)-induced oscillatory contraction in guinea pig trachea. Involvement of PGE₂ in ChTX-induced oscillatory contraction was also investigated. ChTX (100 nM) induced oscillatory contraction in guinea pig trachea. The mean oscillatory frequency induced by ChTX was 10.7±0.8 (counts/h). Maximum and minimum tensions within ChTX-induced oscillatory contractions were 68.4±1.8 and 14.3±1.7 (%) compared with K⁺ (72.7 mM) contractions. ChTX-induced oscillatory contraction was completely inhibited by indomethacin, a nonselective COX inhibitor. VSA, a selective COX-1 inhibitor, did not significantly inhibit this contraction, whereas NS-398, a selective COX-2 inhibitor, abolished this contraction. Exogenously applied arachidonic acid (AA) enhanced ChTX-induced oscillatory contraction. SC-51322, a selective prostaglandin E receptor subtype EP₁ antagonist, significantly inhibited ChTX-induced oscillatory contraction. Exogenously applied PGE₂ induced only a slight phasic contraction in guinea pig trachea, but PGE₂ induced strong oscillatory contraction after pretreatment with indomethacin and ChTX. Moreover, ChTX time-dependently stimulated PGE₂ generation. These results suggest that ChTX specifically activates COX-2 and stimulates PGE₂ production, and that ChTX-induced oscillatory contraction in guinea pig trachea is mediated by activation of EP₁ receptor.