Crude organelle preparations from castor bean (Ricinus communis L.) endosperm rapidly incorporate CDP-(14C)choline and CDP-(14C)-ethanolamine into phosphatidylcholine and phosphatidylethanolamine, respectively. Separation of organelles by sucrose density gradient centrifugation following incubation with these substrates demonstrated that most of the 14C phospholipids thus formed were present in the endoplasmic reticulum membranes, although label was also found in mitochondria, proplastids, and glyoxysomes. The phospholipid-synthesizing enzymes, cholinephosphotransferase and ethanolaminephosphotransferase, are exclusively confined to the endoplasmic reticulum membrane fraction, suggesting that the appearance of 14C-phospholipid in other organelles was due to phospholipid exchange. Phospholipid synthesis was inhibited by the cytoplasmic supernatant fraction. The active inhibitor in this fraction was not identified, but the inhibition was not significantly relieved by either dialyzing or boiling the supernatant. Phosphatidylcholine synthesis showed an absolute requirement for Mg2+; the Michaelis constant was 1 mm. Ca2+ was a potent inhibitor of Mg2+-stimulated phospholipid synthesis and enhanced the decay of 14C-phospholipids from pre-labeled membranes, particularly when the membranes were resuspended in the cytoplasmic supernatant.