A mutation in the parkin gene has been identified as the cause for an autosomal recessively inherited form of Parkinson's disease (PD). The authors have recently isolated the mRNA coding for the rat homolog of parkin and showed its widespread expression in the central nervous system (CNS) by in situ hybridization. In the present study, we investigated the distribution of parkin in the rat CNS with a polyclonal antibody that reacts with a approximately 52-kDa protein, mainly localized in the cytoplasm and corresponding to the predicted molecular mass of parkin. Immunohistochemistry on adult rat brain sections showed a widespread distribution of parkin. This included labeling of cell bodies, nuclei as well as processes in the hippocampus, cerebral cortex, cerebellum, and several nuclei in the brainstem. The regional expression of parkin-immunoreactivity (IR) correlated well with the parkin-mRNA levels assessed by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). This study provides the detailed analysis of the regional and cellular distribution of parkin in the rat brain and may be useful in elucidating its pathophysiological role.