Histochemical investigations of leucine aminopeptidase using LNA (L-leucyl-beta-napthylamide) as a substrate reveals a marked enzyme activity selectively localized to the granular layer with inconspicuous reaction in the stratum moleculare and the Purkinje cells in the rat cerebellum. The LNA-splitting enzyme differs from the well-known leucine aminopeptidase (LAP) by its optimum at pH 5.5. The necessary long incubation period used in the present study, and its focal precipitation of the enzyme reaction product in the same place, like acid phosphatases, in the granular layer, suggest a lysosomal localization. The functional role of the LNA-splitting enzyme has been discussed; it is considered that it is involved not only in the protein transformation for synaptic function, but may perhaps also play an important pathogenic role in necrosis, atrophy or even autolysis.