Long-chain acyl-CoA hydrolase (EC 184.108.40.206), which is found primarily in the brain in rats, catalyzes the hydrolysis of fatty acyl-CoA thioesters. We purified this enzyme, referred to as ACH, from the rat brain cytosol. The molecular masses of the native enzyme and the subunit were estimated to be 104 and 36 kDa, respectively. The enzyme showed high activity with long-chain acyl-CoAs, e.g., with maximal velocity of 262 mumol/min/mg and Km of 5.7 microM for palmitoyl-CoA, but acyl-CoAs with carbon chain lengths of C8-18 were also good substrates. The enzyme was refractory to the inhibitory effect of diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride, but sensitive to p-chloromercuribenzoate. In the rat brain cytosol, about 90% of palmitoyl-CoA hydrolase activity was titrated by anti-ACH antibody, which accounted for over 70% of the enzyme activity found in the brain tissue. Immunoblots of the cytosol prepared from rat brain regional blocks indicated the broad distribution of ACH over the brain, with a relatively high level in the pons and medulla. Immunohistochemically, ACH was localized to neurons. In addition to various nuclei, some neuronal cells, such as mitral cells in the olfactory bulb, pyramidal cells in the cerebral cortex, and Purkinje cells in the cerebellum, were also immunostained with anti-ACH antibody. Brain cytosols prepared from ten mammalian species including human contained a single polypeptide reactive to anti-ACH antibody with molecular masses of 34-36 kDa, together with high activities of palmitoyl-CoA hydrolase. These findings suggest the physiological significance of ACH in the brain, although its precise role remains to be determined.