When bovine kidney mitochondria were assayed in the presence of Triton X-100, they were found to contain glycine N-acyltransferase activity toward the CoA-adducts of benzoate, butyrate, isovalerate, naphthylacetate, phenylacetate, and salicylate. Heptanoyl-CoA activity was masked by high acyl-CoA hydrolase activity. All activities found in detergent-lysed mitochondria, and also that toward heptanoyl-CoA, could be released in soluble form by repeated cycles of freeze-thawing. Activity in the particle-free lysate decreased in the order: phenylacetyl-CoA > benzoyl-CoA > salicylyl-CoA > butyryl-CoA > naphthylacetyl-CoA > heptanoyl-CoA > isovaleryl-CoA. This is quite different from liver, where the activity toward the arylacetic acids is much lower and the other activities are higher. This reflects a major difference in the relative expression of the aralkyl and arylacetyl transferases between liver and kidney. The phenylacetyl-CoA and naphthylacetyl-CoA activity purified with a single protein which is termed the arylacetyl transferase. This enzyme was similar to the hepatic arylacetyl transferase in terms of its sensitivity to sulfhydryl reagents, response to cations, and molecular weight (33,500). Activity toward benzoyl-CoA also purified as a single form which was similar to the hepatic form in its molecular weight (34,000), response to cations, and kinetic properties. Conditions leading to the inhibition of this kidney form and also the hepatic form by p-mercuribenzoate are described.