We have used the isolated perfused lung (IPL) preparation from the rat to determine whether uptake of choline from the vascular compartment could limit the rate of synthesis of phosphatidylcholine (PC). The uptake of choline was rapid and did not saturate at a concentration of 10 mM. The rate of incorporation of choline into phospholipid was saturated above 0.1 mM choline. Whereas, uptake and incorporation were depressed at 4 C, uptake was neither dependent on the extracellular sodium concentration nor inhibited by equimolar concentrations of hemicholinium-3 (HC-3). We could find no evidence that uptake might limit synthesis of lung lecithin and conclude that uptake is either by free diffusion, or by a carrier-mediated process with a very high Km.