The experiments were carried out to observe certain functional changes in the respiratory system which develop in the early stage of aspiration pneumonia and to study the possibility of influencing these changes by application of artificial ventilation (IPPB) and/or administration of dehydrobenzperidol. The experiments were carried out on 32 mongrel dogs divided into 4 groups. Experimental Mendelson's syndrome was produced by instilling, during anaesthesia, hydrochloric acid solution of pH 1.5 in a dose of 4 ml/kg into the tracheobronchial tree. Immediately after anaesthesia and then 10 times at intervals of one hour after HCl instillation the following determinations were done: mean blood pressure, CVP, haematocrit, PO2 and PCO2 in arterial and mixed venous blood, and minute artifical ventilation. The veno-arterial blood shunt in the lungs, alveolo-arterial difference of oxygen partial pressure and the effective compliance were also determined. The chemical damage to the lungs by acid caused condensation of blood and a fall in CVP. These changes were due to fluid escape from the vessels especially in the pulmonary vascular tree. Disturbances in pulmonary gas exchange were a result of deficient ventilation of lung areas damaged primarily by acid and suffering secondarily from developing disturbances in pulmonary blood flow. Dehydrobenzperidol in a dose of 1 mg/kg applied as the only treatment in Mendelson's syndrome had the same favourable effect on pulmonary changes as controlled IPPB. The combination of controlled artificial ventilation and intravenous dehydrobenzperidol decreased the disturbances in pulmonary gas exchange in Mendelson's syndrome. This was possible because both factors exert a benficial effect on ventilation and perfusion preventing the development of oedema and atelectasis.