The distributions of acid and alkaline phosphatases, 5-nucleotidase, ATPase, non-specific esterase, specific cholinesterase, succinic dehydrogenase and beta-galactosidase are described in the mesencephalic auditory, tegmental and cranial nerve nuclei of the frog (Rana tigrina). The main results of the study are as follows: The laminar, principal, and magnocellular nuclei of the torus semicircularis, which are associated with auditory functions, show intense activity of specific cholinesterase. On the other hand, the commissural and subependymal mid-line nuclei, whose functions are doubtful, show a complete lack of this enzyme. The nucleus isthmi shows intense acid phosphatase, ATPase, non-specific esterase, specific cholinesterase and succinic dehydrogenase activities. Non-specific esterase is virtually absent from all the areas studied except the nucleus isthmi and the 3rd and 4th cranial nerve nuclei. Most of the commissures and fibre tracts show intense activity for beta-galactosidase and 5-nucleotidase. The possible roles of these enzymes in glycolipid and myelin metabolism are discussed.