A technique by which drug access was restricted to either the mucosal or the adventitial surface of tracheal rings isolated from sensitized guinea-pigs was applied to study the role of the epithelium in modulating responses to KCl, acetylcholine, histamine and antigen (bovine serum albumin, BSA). Epithelium removal did not alter the responsiveness or sensitivity of tracheal rings to KCl. In contrast, a leftward shift occurred for concentration-response curves to acetylcholine (concentration ratio (CR) = 4.1), histamine (CR = 2.9) and BSA (CR = 33.9) entering from the mucosal surface of de-epithelialized trachea. This shift was not associated with changes in the maximal effect of the spasmogens. Response to the adventitial entry of these spasmogens was not altered by epithelial stripping. These results confirm that the epithelium modulates tracheal responses to certain spasmogens including antigen challenge. This role was exclusively exerted for mucosal drug entry. The mechanism underlying this protective effect of epithelium remains to be determined.