Abstract: The aim of this study is to determine the effect of levamisole and levamisole-trichlorfon combination on isolated sheep trachea. Contraction was achieved with levamisole concentrations (10-8-10-3 M) on tracheal strips of adult sheep (>1 year of age). Pretreatment with trichlorfon (10-7 M)-levamisole (10-7 M) decreased the pD2 and Emax of Ach when compared to levamisole pretreatment (p<=0.01). Pretreatment with levamisole (10-7 M), decreased the Emax (p<=0.01) and pD2 (p<0.05) of bethanechol concentrations (10-8-10-3 M) significantly. Pretreatment with atropine (10-6 M) decreased the Emax of levamisole (10-4 M) significantly (p<0.05). To conclude, levamisole acted mainly on the muscarinic receptors of the sheep trachea and this effect was partly inhibited by atropine. Adverse interaction was present between levamisole and trichlorfon.