Endotoxin protects against pulmonary oxygen toxicity in rats, and both prostaglandins and polymorphonuclear leukocytes (PMN) are implicated as playing an important role in this protective action. In this study, a bronchoalveolar lavage (BAL) technique was used to analyze cellular and eicosanoid composition of the lavage fluid of endotoxin-protected oxygen-exposed rats. The BAL fluid of the endotoxin-protected oxygen-exposed rats contained the highest number of PMN, while the BAL fluid of the nonprotected oxygen-exposed rats contained the highest number of macrophages. Thus, morbidity due to pulmonary oxygen toxicity was correlated with the number of macrophages but not with the number of PMN present in the BAL fluid. Leukotriene B4, thromboxane B2, and prostaglandin E2 levels were significantly higher in the lavage fluid of nonprotected oxygen-exposed rats compared to the levels in the lavage fluid of air-exposed rats. Eicosanoid levels in the BAL fluid of endotoxin-protected oxygen-exposed rats did not differ significantly from the levels found in air-exposed control rats. These findings suggest that endotoxin protects against hyperoxia-induced changes in eicosanoid metabolism.