The endogenous phospholipid acceptor for Ca2+-dependent serine-phospholipid base-exchange reaction was explored in rabbit platelet membranes. When chasing experiments were carried out using prelabeled membranes, cold serine chased labeled serine and ethanolamine, but not labeled choline from corresponding phospholipids. This indicates that phosphatidylserine (PS) and phosphatidylethanolamine (PE) are at least utilized as phospholipid acceptors for serine base-exchange reaction, although it is not certain whether phosphatidylcholine (PC) is utilized. For the quantitative analysis, the amounts of endogenous bases released from the membranes during serine base-exchange reaction were measured. The serine-dependent release of ethanolamine and choline was only about 10 and 3 % of the amount of [3H]serine incorporated into PS, respectively. If Ca2+-dependent serine incorporation into PS occurs only through the so-called base-exchange reaction, these results suggest that the major phospholipid acceptor for serine incorporation is preexisting PS, but not PE or PC in rabbit platelet membranes.