The purpose of this study was twofold: (1) to analyze the cellular components of bronchoalveolar lavage fluid throughout the development of oleic-acid-induced lung injury in the rat and (2) to investigate the role of polymorphonuclear leukocytes (PMN) in the pathogenesis of this disease. Animals were killed and lavaged at various times after a single intravenously administered injection of oleic acid. The results demonstrate that a significant influx of inflammatory cells appear in the lavage fluid as early as 4 h after the administration of oleic acid. The PMN are the first cells to appear, and significant levels persist through Day 5 after injection. There is a transient yet significant influx of lymphocytes between 3 and 7 days after treatment. Rats treated with oleic acid displayed significant increases in lung vascular permeability over control animals at 1 and 4 h after injection. Depletion of PMN by anti-PMN serum significantly decreased the permeability changes induced by oleic acid. Treatment of oleic-acid-injected animals with catalase, superoxide dismutase, or dimethyl sulfoxide failed to inhibit lung permeability changes induced in this model.