Astract. Oxygen free radicals released during endotoxemia may contribute to the lung injury of the adult respiratory distress syndrome (ARDS). As this syndrome occurs frequently after gram-negative sepsis in humans, we studied the effect ofintravenous N-acetylcysteine (NAC), a free radical scavenger, upon the endotoxin (E)-induced model of ARDS in awake sheep. In vivo studies demonstrated that NAC attenuates the endotoxin-induced rise in pulmonary artery pressure (62±3 torr with E control vs. 43±3 torr for E + NAC), and markedly diminishes the rise in lymph flow at 1 h (8.5±1.2 vs 4.5±0.6 ml/15 min) and 4 h (5.0±0.6 vs. 3.3±0.4 ml/15 min), respectively, for E control vs. E + NAC. NAC also markedly attenuated the alterations in lung mechanics after endotoxemia. Dynamic compliance at 2 h after endotoxemia was 44±6% of base line for E vs. 76±10% of base line for E + NAC. Resistance to airflow across the lung at 1 h postendotoxin was 811±280% of base line for E vs. 391±233% of base line for E + NAC. NAC substantially reduced the 1 h postendotoxin rise in lymph concentrations of thromboxane B2 (8.29±3.28 vs. 2.75±1.93 ng/ ml for E vs. E + NAC) and 6-keto-prostaglandin-Fja (0.91±0.27 vs. 0.23±0.12 ng/ml for E vs. E + NAC). In addition, in vitro studies were performed which revealed NAC to be a potent free radical scavenger in both biologic and nonbiologic free radical generating systems. NAC decreased phorbol-stimulated granulocyte aggregation in a concentration-dependent manner in vitro. Minimal ef-

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