Incorporation of [ 1 4 C]serine into phosphatidylserine by the base exchange reaction in Ehrlich ascites tumor cells is inhibited by 30% by glucose and by 60% by 2-deoxyglucose (10 mM each). The inhibition by thapsigargin (0.2 μM) amounts to 80%. This inhibition is interpreted as being due to depletion of calcium stores in the endoplasmic reticulum and is compatible with the previous observation [Teplova, V.V., Bogucka, K., Czyz, A., Evtodienko, Yu. V., Duszynski, J., and Wojtczak, L. (1993) Biochem. Biophys. Res. Commun. 196, 1148-1154] that glucose and deoxyglucose elicit an increase of cytoplasmic [Ca 2 + ] at the expense of intracellular Ca 2 + stores and with the finding [Baranska, J. (1989) FEES Lett. 256, 33-37] that the base exchange reaction requires high concentration of Ca 2 + within the endoplasmic reticulum lumen.