The pulmonary vasculature and perivascular tissue is able to generate arachidonic acid metabolites with strong effects on vascular tone and permeability. Stimulators of the pulmonary arachidonic acid metabolism, which represents a highly potent mediator system, are components of the classical cascade systems, bacterial toxins, hypoxia, and a variety of other physiological and non-physiological factors. Upon excessive stimulation, the pulmonary circulation responds with vasoconstriction and extravasation. Since granulocytes are always involved, their contribution to the resulting vascular effects has to be considered.