Aldehyde fixatives containing high concentrations of glutaraldehyde, usually used for conventional electron microscope studies, were successfully used for immunocytochemistry of transmitter synthesizing enzymes, glutamate decarboxylase and tyrosine hydroxylase, in the rat central nervous system. Although a high concentration of glutaraldehyde could cause tremendous non-specific staining, this was almost completely absent after treating sections with 1% sodium borohydride for 30 min. Furthermore, it was shown that a high concentration of glutaraldehyde might cause no appreciable reduction of the antigenicities of glutamate decarboxylase and tyrosine hydroxylase when compared with fixatives containing a low concentration of glutaraldehyde. It is suggested that fixatives containing high concentrations of glutaraldehyde are very useful, not only for conventional electron microscope studies, but also for light and electron microscope immunocytochemistry of some antigens, including glutamate decarboxylase and tyrosine hydroxylase.