The mechanisms for accumulation of disaturated phosphatidylcholine (PC) molecular species in developing fetal guinea-pig lung during the period of surfactant synthesis, between day (d) 55 and term (d68), were determined by the incorporation of 50 mu Ci [methyl-14C]choline into lung PC in utero over 3 h. Comparison of the pattern of PC synthesis de novo with the composition of the total PC pool indicated that approx. 50% of the total PC16:0/16:0 was synthesized by acyl remodelling of PC16:0/18:2 by the actions of phospholipase A2 and acyltransferases. Acyl remodelling was established before the onset of surfactant synthesis (d55) and so was not specific for this process. Between d55 and term the concentration of lung PC increased significantly. Conversely, the incorporation of [14C]choline into lung tissue and the rate of PC synthesis decreased over this period. Calculation of turnover times of lung PC species suggested that the increase in lung disaturated PC concentration during surfactant production might be due to a differential decrease in catabolism rather than increased PC synthesis.