The pathophysiology of neonatal meconium aspiration syndrome (MAS) is related to mechanical obstruction of the airways and to chemical pneumonitis. It has also been suggested that meconium causes inhibition of surfactant function. To assess its in vitro effect on surfactant function and morphology, we used a pulsating bubble surfactometer to measure the dynamic surface tension of meconium-surfactant mixtures and observed their electron microscopic structures. The mixtures were prepared by adding serial dilutions of human meconium to various concentrations of Surfactant-TA (Surfacten) that had been used for the prevention and treatment of neonatal respiratory distress syndrome. Inhibition of the surface tension-lowering properties of Surfactant-TA was caused by the addition of meconium and depended on the concentration of the surfactant; the inhibition could be overcome by increasing the surfactant concentration. When meconium was added to Surfactant-TA, the characteristic ultrastructural features of the latter, the loosely stacked layers, changed to a spherical lamellar structure and folded linear structures. These results suggest that meconium inhibits surfactant function by altering surfactant morphology. Our morphologic and functional findings support the new concept that surfactant inhibition may play a role in the pathophysiology of MAS.