Serial measurements of pulmonary function and arterial blood gases during the first 3 postnatal days of life were obtained in 12 infants with meconium aspiration syndrome (MAS). Nine normal neonates with similar weight and gestational age were studied as controls. Infants with MAS has significantly lower pH on day 1, and had greater P(A-a)O2 throughout the study period than that of normal controls. The PCO2 was comparable between the groups. Both dynamic lung compliance (Cdyn) and specific lung compliance (C/VL) were lower in infants with MAS as compared with those of normal infants. The functional residual capacity (FRC) for normal infants on days 1, 2, and 3 were 2.0 +/- 0.3, 2.1 +/- 0.3, and 2.2 +/- 0.3 ml/cm, respectively, and for infants with MAS were 1.8 +/- 0.4, 2.3 +!- 1.1, and 2.2 +/- 0.6 ml/cm, respectively. Radiographic hyperinflation of the lungs was seen in 6 infants with MAS on day 1; 3 were associated with high FRC (greater than 2 SD of normal) and 2 with low FRC, indicating air trapping. The early use of PEEP should be cautious if hyperinflation or air trapping is present.