We exposed rats to 4% CO2-10% O2-86% N2 for 24 b before infusing with 20 μCi kg-1 (methyl-3H)cboline chloride. They were then exposed for periods up to 48 b, at which times their lungs were degassed and lavaged. The lavage fluid was divided into a tubular myelin-rich (PLalv-1) and a tubular myelin-poor (PLalv-2) fraction. Lamellar body (Ib) and microsomal (m) fractions were prepared from the lung tissue and the amount of phospbolipid (PL) was determined in each of the 4 fractions. Specific activity (sp.act.) curves were constructed for both control and byperpneic groups. Exposure to the gas doubled both tidal volume and frequency of breathing. Total PLalv, PLlb and PLm were all markedly elevated, which, when taken in conjunction with the changes in sp.act. in these fractions, suggests that the rate of surfactant PL synthesis was increased within 24 b. The shapes of the sp.act. curves suggest precursor-product relationships between PLlb and PLalv-1 and between PLalv-1 and PLalv-2. However, when we applied analysis based on the Zilversmit steady-state equation, instead of the expected straight line, we found a marked clockwise hysteresis that did not return to the origin. Whereas this may reflect PLalv being supplied from 2 tissue pools, we argue that, in fact, we are not dealing with classic compartmental precursor-product relationships.