Pulmonary intravascular macrophages (PIMs) have been recognized as the site of substantial uptake of blood-borne particles in the lungs of a number of domestic animal species. Concomitantly, there is a pronounced lung susceptibility to endotoxin in calves, goats, sheep, pigs, and cats. Hemodynamic changes and initial lung injury after endotoxin administration are mediated by arachidonic acid metabolites from a pulmonary source. A significant role of PIMs in regulating pulmonary hemodynamics is implicated.