The capacity to form ubiquitin (Ub)-protein conjugates was investigated in the cytosol of different structures of the rat central nervous system (CNS) in order to confirm the presence of this extralysosomal, adenosine triphosphate (ATP)-dependent, protein degradation system as well as its structural localization. Using '251-Ub, we found that in the presence of ATP, the cytosol obtained from whole brains was able to form high molecular weight Ub-protein conjugates. These conjugates could be detected after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and radioautography. The formation of these conjugates was much higher in the cerebral cortex than in the brain stem, which is mainly constituted by white matter, being intermediate in the cytosol isolated from whole brain total homogenates. These results suggested to us that under normal conditions the capacity to form Ub-protein conjugates was mainly located in structures containing neuronal cell bodies. Strong support for this contention was obtained when the cytosol isolated from rat optic nerves or from oligodendroglial cells isolated from whole brain was found to be totally unable to form Ub-protein conjugates. The inability of certain CNS structures to form conjugates with Ub could be attributed, among other reasons, to the lack of enzymes catalyzing the various steps of the Ub degradation system, to the absence of short half-life (target) proteins in those structures, or to the lack of activity of the enzymes catalyzing the reaction due to regulatory control mechanisms operating under normal conditions.