BACKGROUND: Oxygen free radicals are likely to be involved in decreases of the tracheal epithelial barrier function, increases of permeability, and inhibitions of ciliary activity. The present study was undertaken to determine the interaction between isoflurane, propofol and oxidative injury with respect to the contractile force of tracheal smooth muscle in guinea-pig. METHODS: Strips of guinea-pig trachea were suspended in organ chambers, and their isometric tension was recorded by a MacLab. Tissues were allocated to 7 groups (each: n = 10) of control, 2%, 3%, and 4% isoflurane, 25 micrometer, 50 micrometer, and 100 micrometer propofol. All strips were challenged with 10 5 M acetylcholine (ACh) to get maximal contractions and followed washout. All strips were exposed to 10 4 M H2O2 contained modified Krebs solution for 30 minutes after the strips were perfused with each concentration of anesthetics for 20 minutes. After washout of organ chambers and 30 minutes of rest, all strips were contracted with ACh. Several strips were prepared for microscopic evaluation. RESULTS: The contractile heights to H2O2 showed 36.7+/-20.2% of control value in the control group, and there were significant differences between the control and the propofol 100 micrometer group as shown by Tukey test. There were no significant differences in contractile heights to the second ACh in any of the 6 groups except the 100 micrometer propofol group. Microscopic morphological changes were not detected by 1,000 light microscopic evaluation. CONCLUSIONS: We suggest that the contractile heights of strips to H2O2 which were lower in four anesthetic groups than in the control group indicated a counteracting relaxation of smooth muscle caused by anesthetics. We suggest that there might be some functional effects of H2O2 on smooth muscle cells other than epithelial injury and that 100 micrometer propofol might have some protective effects against smooth muscle cell injury from 10- 4 H2O2.