In the present study the pathogenesis of the pulmonary damage following infusion of thrombin in combination with a fibrinolysis inhibitor, AMCA, in the dog was elucidated. An important mechanism in the development of the pulmonary damage following infusion of thrombin and AMCA seems to be an increased vascular permeability in the pulmonary microvasculature leading to pulmonary oedema. The question whether this pulmonary damage can be prevented by antihistamine (mepyramine maleate), antiserotonins (methysergide, reserpine) antiprostaglandins (acetylsalicylic acid, indomethacin, polyphloretin phosphate), 'anti-inflammatory agents' (methylprednisolone, calcium) or an anti-adrenergic agent (phenoxybenzamine) was investigated. None of these agents did prevent the lung damage following thrombin and AMCA. In order to study the possible role of bronchoconstriction, the complement system and the kinin system for this damage dogs were also artificially ventilated with an increased end-expiratory pressure, decomplemented with cobra venom factor or treated with Trasylol respectively. Neither were these treatments effective in preventing the pulmonary damage. The findings of the present study suggest that the permeability increasing substance involved in the pathogenesis of the pulmonary damage following thrombin and AMCA is not histamine, serotonin, prostaglandins or bradykinin. Therefore another, still unknown factor, may be of greater importance for this damage.