To determine the ultracytochemical localization of monoamine oxidase (MAO) activity of normal rat choroidal epithelium, the cerium method was used. The electron dense reaction products of MAO activity were found only along the external membrane of mitochondria. A modified fluorescent Faglu method was also employed and catecholamine nerve fibers were identified along vessels of the choroid plexus.
The possible relation between MAO and catecholamine nerve fibers was briefly discussed.