Sensitized guinea pig tracheal preparations with epithelium removed were more sensitive to acetylcholine, histamine substance P, and barium chloride than those with epithelium intact. EC50 of them obtained in the preparations with epithelium removed dramatically decreased to 1/6-1/30 vs that obtained in epithelium intact ones. The amplitudes of contractions of epithelium-removed preparations induced by antigen-antibody reaction or electric field stimulation increased by 50%-100% vs the control. These results indicated that the epithelium had important modulative effects on the airway, especially on its allergic contractions.