Hypertonic solutions induce obstruction in asthmatic patients. Hypertonic NaCl and KCl evoke epithelium-dependent relaxation of guinea-pig isolated, perfused trachea (IPT), a preparation in which agents can be added separately to the basolateral (extraluminal, EL) or mucosal (intraluminal, IL) bath. We compared responses of EL-methacholine(3 x 10-7 M)-contracted IPT to hypertonic mannitol, N-methyl-D-glucamine (NMDG) and urea. Mannitol and NMDG added to the EL or IL bath elicited relaxation responses, being more potent in the IL bath. EL mannitol elicited contractile responses in lower concentrations in many preparations, and relaxation was followed by contraction in higher concentrations of EL mannitol and NMDG. Likewise, EL urea caused contraction in low concentrations and contractile responses to higher concentrations were preceded by relaxation; IL urea caused only relaxation. Both the EL-contractile and the IL-relaxant responses to urea were attenuated in tracheas from which the epithelium was removed. The possibility that organic osmolytes involved in cell volume regulation could mediate the contractile or relaxant responses was examined. In tracheal strips, taurine, sorbitol, myo-inositol and betaine did not initiate contractile or relaxant responses. Our findings suggest that hypertonicity elicits complex responses in the airways which are agent-specific, epithelium-dependent, polarized across the epithelium, and not mediated by organic osmolytes.