Contractile and relaxant responses of guinea-pig tracheal smooth muscle can be measured in vitro using the isolated, perfused trachea (IPT) preparation, which allows agents to be added separately to the serosal (extraluminal; EL) or mucosal (intraluminal; IL) surfaces. In response to hyperosmolarity at the EL or IL surfaces, the epithelium releases EpDRF which relaxes the airway smooth muscle. Previously, we have shown in IPT precontracted with EL methacholine (3x 10^-7 M) that LPS (4 mg/kg, i.p.; 18 h) potentiates EpDRF-mediated smooth muscle relaxation in response to elevation of IL osmolarity with NaCl. The purpose of this study was to determine if potentiation of smooth muscle relaxation by EpDRF following LPS-treatment is dependent on the osmolyte used. When KCl and urea were used, there were no differences in maximum relaxation responses between control- and LPS-treated groups; however, the EC50 for urea-induced relaxation was increased after LPS-treatment. Relaxation responses to mannitol and NaCl were potentiated after LPS treatment, whereas the EC50's were not changed. These results suggest that the effect of LPS-treatment on the release and/or effects of EpDRF are dependent on the osmolyte used to elevate IL toxicity.