In animals oleic acid induces a form of lung injury similar to that observed following fat embolization in humans. In the present study, canine lower left lung lobes were isolated, ventilated, and perfused at constant pressure with heparinized, autologous blood. Weight gain, in the absence of vascular volume change, in this preparation is a sensitive indicator of edema. One group of lobes was pretreated with 5.1 mg/gm lobe wt methylprednisolone before 1 μl/kg body wt oleic acid. An untreated group given only oleic acid served as controls. Following oleic acid, the reduction in effective lobe compliance and blood PO2and the increase in perfusate total protein concentration were similar in both groups. Rate of lobe weight gain following oleic acid was remarkably linear in both groups but significantly lower (p< 0.05) in lobes pretreated with methylprednisolone. The latter group showed a total weight gain of 21% compared to 34% in the controls 3 hr after oleic acid challenge. We conclude that oleic acid induces a pulmonary edema in the absence of an elevation in hydrostatic pressure. Furthermore, rate of edema formation is attenuated by methylprednisolone pretreatment.