To further define the role of platelet-activating factor (PAF) in endotoxin-induced lung dysfunction, we examined the effect of ABT-299, a specific and potent PAF-receptor antagonist, on the response to endotoxemia in six chronically instrumented awake sheep. We administered Escherichia coli endotoxin (0.5 µg/kg) intravenously with or without pretreatment with ABT-299 while monitoring mean pulmonary arterial pressure (Ppa), mean systemic arterial pressure (Psa), dynamic compliance of the lungs (Cdyn), and functional residual capacity (FRC). Endotoxin administration caused pulmonary hypertension, reduced Cdyn, leukopenia, and hypoxemia while having no significant effect on Psa or FRC. Administration of ABT-299 did not affect any of the measured variables at baseline. Pretreatment with ABT-299 attenuated the peak Ppa seen after endotoxin administration but had minimal effects on endotoxin-induced changes in Cdyn, white blood cell count, or alveolar-to-arterial oxygen difference. ABT-299 was shown to completely block the pulmonary hypertension and reduction in Cdyn seen after intravenous administration of exogenous PAF. We conclude that PAF does not play an essential role in the sheep's response to endotoxin.