Abstract Rat brain synaptosomes prelabeled with [ 14C]arachidonoyl-phospholipids were used to study the characteristic properties of acyl hydrolases for different phospholipids. Incubation of the prelabeled synaptosomes at 37°C resulted in a time-dependent decrease of label from phosphatidylcholines (PC) and phosphatidylinositols (PI) and a concomitant increase in label in the free fatty acid fraction, but not diacylglycerols (DG). Phosphatidylserines (PS) also showed a decrease in radioactivity, but little change was observed for phosphatidylethanolamines (PE). At pH 7.4, the release of labeled arachidonate from PI was Ca 2+-dependent, but that from PS and approx 50% of that from PC was not. The hydrolysis of PC was greatest at pH 7.4, but Ca 2+-dependent hydrolysis of PI was active from pH 5.5 to 8.5. All detergents tested severely inhibited the release of labeled arachidonate, but in the presence of Ca 2+ and deoxycholate or taurocholate, a large portion of PI was converted to DG through activation of the PI-phosphodiesterase. Different effects on the phospholipid hydrolysis were observed with different phospholipase A 2 inhibitors. Mepacrine (1 mM) inhibited the Ca 2+-dependent hydrolysis of PI but not PC, whereas dibucaine (1 mM) inhibited PC hydrolysis by 40% but did not affect PI. p-Bromophenacyl bromide (1 mM) dissolved in dimethylsulfoxide (DMSO) only partially inhibited (about 40%) the hydrolysis of PI and PC. The preferential hydrolysis of PI and PC by endogenous phospholipid acyl hydrolase correlates well with the observation that these same two lyso-phospholipids are also preferred by the acyltransferase for the reacylation process.