Surfactant protein B (SP-B) is critical to the biophysical function of surfactant. To characterize its metabolism in vivo in the newborn, we administered [ 3 5 S]methionine and [ 3 H]palmitate to newborn rabbits intravascularly. Three groups of 4 rabbits per group were killed at each of 4 time points followed by isolation of SP-B from alveolar wash and lamellar bodies. The labeling kinetics for alveolar wash associated SP-B and saturated phosphatidylcholine (Sat PC) had similar patterns. To characterize SP-B clearance from the airspace, rabbit SP-B was iodinated, mixed with [ 1 4 C]dipalmitoylphosphatidylcholine and given by intratracheal injection. Alveolar washes and lamellar bodies were recovered from 4 animals at each of 7 time points. Both SP-B and Sat PC were cleared slowly from the total lung (half-life values 25 h). However, SP-B was cleared more rapidly from the airspaces than was Sat PC. The ratio of [ 1 2 5 I]SP-B to [ 1 4 C]Sat PC in lamellar bodies increased 2-fold by 8 h. These results support the concept of linked secretion and clearance pathways for SP-B and Sat PC, although small differences in reuptake were detected.