Subcellular distribution of soluble and membrane-bound Leu-, Arg- and Asp-beta-naphthylamide hydrolysing activities (arylamidase activity) was studied from left and right rat brains, each including hemisphere, cerebellum and brain stem. Both soluble Leu- and Arg-beta-naphthylamide hydrolysing activities showed the highest levels in the synaptosomal fraction. However, the microsomal fraction presented the highest levels when membrane-bound activity was assayed. When we used Asp-beta-naphthylamide as substrate, there were no differences among fractions in the membrane-bound activity, and the highest soluble activity was present at the mitochondrial level. Two different patterns in the subcellular distribution of enzymatic activity were observed: One of them was the result of the use of Leu- or Arg-beta-naphthylamide as substrate and the other when Asp-beta-naphthylamide was employed. No differences between left and right brains in soluble or membrane-bound activities were found.