ACETYLCHOLINE stimulates the turnover of phosphatidylinositol (PI) and phosphatidic acid (PA) in brain in vitro and in vivo2,3. It has been suggested that this effect on phospholipid metabolism is related to synaptic nerve pulse conductance4,5, and that phospholipid exchange proteins transfer PI from the site of synthesis—the endoplasmic reticulum—to the synaptosomal plasma membrane5. Exchange proteins have been isolated from bovine brain that transfer PI to membranes deficient in this phospholipid in vitro6–8. These proteins also catalyse the transfer of phosphatidylcholine (PC) and PA, although to a lesser extent7. We report here the presence of phospholipid exchange proteins in the soluble protein fractions derived from a synaptosomal membrane and myelin fraction.