SummaryThe relationship between pulmonary function and the migration of meconium to distal airways was determined in 10 rabbits (mean weight 2.6 kg) after insufflation of a meconium‐saline mixture (1‐2 ml/kg). Animals were anesthetized, cannulated, intubated, and mechanically ventilated with 100% oxygen. Lung mechanical dysfunction was most severe during the early phase of meconium migration, 15 min postinsufflation. Substantial increases in inspiratory lung resistance (RI) and expiratory lung resistance (RE) suggest that the site of obstruction at 15 min was the large airways. A decrease in dynamic lung compliance with unchanged static compliance characterizes the obstruction as partial. At 15 min and throughout the migration process, REwas greater than RI, demonstrating a check‐valve effect. This phenomenon was substantiated by an increased functional residual capacity (FRC) in all rabbits, presumably due to gas trapping. Secondary to these changes, marked hypoxemia, hypercapnea, and acidosis developed in spite of assisted ventilation with 100% oxygen. At 60 and 120 min postinsufflation, both RIand REdecreased as compared to 15 min. This suggests that the predominant site of obstruction shifted to medium and small airways concomitant with the migration of meconium. Widespread and uneven distribution of meconium still produced significant frequency dependence of lung compliance. Static compliance remained unchanged, indicating that meconium does not affect surface‐active or tissue properties of the lung within 120 min postinsufflation. These data suggest that effective respiratory management after meconium aspiration is dependent on the degree of meconium migration, as reflected by pulmonary mechanics.SpeculationSignificant effects of meconium aspiration on lung function are attributable to migrating obstructive phenomena, which result in a more severe lung dysfunction in earlier stages postinsufflation of meconium than in progressively later stages. It is appealing to speculate that noninvasive pulmonary function measurements made in newborn infants may quantitate the degree of mechanical obstruction and meconium migration in the airways. This information would provide guidelines for effective respiratory management of infants with meconium aspiration syndrome (MAS). Based on the present animal model, elevated oxygen therapy should be employed early to offset the severe hypoxemia. In addition, the check‐valve effect (RE> RI) indicates the use of a low I/E ratio in assisted ventilation. It has been shown that continuous positive airway pressure improves arterial oxygen tension in some newborns with MAS. However, in light of the mechanical dysfunction resulting in an increased FRC, caution is recommended in the use of end distending pressure within 2 hr after meconium aspiration.