The authors examined 152 children with congenital heart diseases complicated by pulmonary hypertension. 109 patients were operated on under conditions of hypothermic perfusion and 43--under deep hypothermia with extracorporeal circulation. Combination of disturbed mechanics of respiration and gas exchange was determined as the respiratory distress syndrome. The initial value of lung expansibility was found to be in inverse proportion to the degree of pulmonary hypertension. With the increase of pulmonary hypertension lung expansibility reduced before extracorporeal circulation and gas exchange became worse. Hypothermic perfusion at volumetric rates of 1.7-2.55 l/min/m2 led to diminished lung expansibility in the early postperfusion period of the operation, the mechanics and gas exchange grew worse. In patients who underwent operation under conditions of deep hypothermia the values of lung expansibility and gas exchange were stable. The incidence of the respiratory distress syndrome was 56.3% after hypothermic perfusion and 7.2% after deep hypothermia.