Immature newborn rabbits were treated at birth by tracheal instillation of porcine surfactant (100 microliters, phospholipid concentration 80 mg.ml-1), to which [14C]dipalmitoylphosphatidylcholine had been added as a marker. They were kept in a body plethysmograph/pneumotachygraph system at 37 degrees C. During a 120 min period of artificial ventilation with a peak insufflation pressure of 20 cm H2O, there was a gradual decrease in tidal volumes (36%.h-1). This decrease was correlated to an elevation of minimum surface tension (r = 0.81; P less than 0.01) and to a prolongation of the adsorption rate (r = 0.80; P less than 0.01) of surfactant recovered by lung lavage from the same animals. There was also correlations between duration of ventilation and minimum surface tension (r = 0.56; P less than 0.01), and between duration of ventilation and adsorption rate (r = 0.73; P less than 0.01). The surface properties of phospholipids extracted from the lavage fluid were similar to those of the original surfactant preparation. Our data suggest that, in immature newborn rabbits subjected to artificial ventilation, exogenous surfactant may become inactivated, probably due to protein leakage into the airspaces.