Recent studies have suggested that loaded breathing elicits alterations in diaphragmatic glutathione levels that may be mediated by free radicals and may also be linked to the development of diaphragm fatigue. While free-radical generation in a number of pathophysiologic conditions is known to be a function of ambient oxygen concentrations, the effect of varying inspired oxygen concentration on the diaphragmatic response to loaded breathing (i.e., on diaphragm fatigue and glutathione levels) has not been studied. In this study, we compared the effect of loaded breathing, continued until respiratory arrest in decerebrate rats breathing room air (RA), with the effect of the same load on animals breathing 100% oxygen (O2). After arrest, the animals' diaphragms were excised, force generation was assessed in vitro, and diaphragmatic levels of reduced glutathione (GSH) and oxidized glutathione (GSSG) were determined. Similar measurements were made on unloaded control animals. We found both similarities and differences in the response to loading in O2- and RA-breathing animals. O2-breathing loaded animals had a greater load endurance, lower blood pressure at the end of loading, higher carbon dioxide levels, and greater high-frequency fatigue at the conclusion of loaded trials than did RA-breathing animals. The degree of low-frequency fatigue was similar, however, in the O2- and RA-breathing loaded groups (i.e, twitch force averaged 7.9 +/- 0.6, 8.4 +/- 0.5, 3.8 +/- 0.9, and 4.5 +/- 0.8 N/cm2, respectively, in the RA/unloaded, O2/unloaded, RA/loaded, and O2/loaded groups, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)