The gastric juice aspiration syndrome (GJA-S, Mendelson syndrome) was studied experimentally in pigs. Following instillation of gastric juice into the right main bronchus necrosis of pneumocytes and bronchiolar epithelium occurred with activation of complement and a prostaglandin E releasing system (possibly the kinin system). Cell necrosis was followed by loss of surfactant and formation of hyaline membranes, rich in immunoglobulin M. The alveolar damage organized, resulting in intraalveolar and interstitial fibrosis. The causative agents were found to be both gastric hydrochloric acid and pepsin. Pretreatment with H2-, or acetylcholine-receptor-antagonists (cimetidine or pirenzepin) as well as buffering of the gastric juice to a neutral pH did not prevent lung fibrosis. If a mixture of aluminium hydroxide, magnesium carbonate and oxethazaine was added to the aspirate, development of lung fibrosis was prevented, but severe granulomatous reaction with foreign body giant cells within both lungs evolved. Kallikrein inhibitor, when administered intravenously not later than 3 min after artificial aspiration, protected the left lung completely and large areas of the right. If infused within 60-90 s complete protection of the left lung and the right upper lobe was achieved. In the majority of the animals a mild focal fibrosis developed in the right lower lobe; in one experiment both lungs were devoid of fibrotic areas. If Kallikrein inhibitor was infused 5 min prior to aspiration, lung fibrosis was not prevented.