Biochemical and morphometric techniques were combined to estimate the distribution and flux or rat lung disaturated phosphatidylcholine (DSPC) in surfactant pathways. We found 126 +/- 8 X 10(6) alveolar epithelial type II (AEP II) cells per lung, each containing 150 +/- 30 lamellar bodies (LB). High yields of LB were isolated by centrifugation of lung homogenates with a discontinuous sucrose density gradient. Measurement of DSPC in LB and alveolar lavage (LV) material demonstrated that only 30% (LB 14%, LV 16%) of lung DSPC was present in these surfactant compartments. Radiolabeling of lung DSPC with [14C]palmitate demonstrated a product-precursor-relationship between LB and LV and a surfactant-associated DSPC flux of 90 microgram-lung-1 X h-1. We conclude that, although only 16% of lung DSPC is in an intracellular surfactant pathway at any time, all this DSPC is ultimately secreted onto the alveolar surface.