N- Acylphosphatidylethanolamine , incubated with dog brain homogenate or microsomes, was hydrolyzed to phosphatidic acid and N-acylethanolamine by a phosphodiesterase of the phospholipase D type. In the absence of F-, phosphatidic acid was further hydrolyzed to diacylglycerol and Pi while N-acylethanolamine was hydrolyzed by an amidase to fatty acid and ethanolamine. The phosphodiesterase showed an alkaline pH optimum and was also active towards N- acetylphosphatidylethanolamine , N-acyl-lysophosphatidylethanolamine, and glycerophospho (N-acyl)ethanolamine but showed little activity toward phosphatidylethanolamine and phosphatidylcholine. Ca2+ stimulated slightly at low concentrations but inhibited at higher concentrations. Triton X-100 stimulated the hydrolysis of N- acylphosphatidylethanolamine , inhibited that of N-acyl-lysophosphatidylethanolamine and glycerophospho (N-acyl)ethanolamine, and had no effect on phosphatidylethanolamine or phosphatidylcholine hydrolysis. The N-acylethanolamine hydrolase (amidase) was also present in the microsomal fraction and exhibited a pH optimum of 10.0. In addition to hydrolysis by the phosphodiesterase, N- acylphosphatidylethanolamine was also catabolized by microsomal phospholipases A1 and/or A2 to N-acyl-lysophosphatidylethanolamine, some of which was further hydrolyzed to glycerophospho (N-acyl)ethanolamine.