The effect of lanthanum on lung phospholipid secretion stimulated by lung distension was examined in 29.5-d gestation newborn rabbits. Lung lavage with saline alone produced 608 +/- 109 micrograms phospholipid/lung (mean +/- SEM), whereas lavage with 10(-3) M lanthanum produced only 153 +/- 40 micrograms phospholipid/lung (p less than 0.05), the difference in yield being attributed to inhibited lavage-induced secretion with lanthanum. If this interpretation is correct, then an increased residual tissue surfactant should be found in lanthanum-lavaged lungs. Postlavaged lung was separated into IA, IB, and IC fractions by homogenization and then centrifugation with 0.25 and 0.65 M sucrose. The surfactant IB fraction was 763 +/- 65 micrograms phospholipid/g wet lung for saline-lavaged lungs, but 1002 +/- 90 micrograms phospholipid/g wet lung for lanthanum-lavaged lungs (p less than 0.002). This confirms that decreased secretion with lanthanum was associated with increased residual surfactant in postlavaged lung. It is concluded that lanthanum is a potent inhibitor of surfactant secretion stimulated by lung distension.