Exposure to heat shock and other stressful conditions activates in cells of all organisms a specific genetic program. This enhances the synthesis of proteins with a protective role against cellular damage, called heat shock proteins (hsps). Furthermore, in the mammalian nervous system, a considerable amount of hsps is also synthesized under normal conditions suggesting that they play an important role in the metabolism of unstressed cells. In this study we analysed the constitutive expression of proteins belonging to the hsp70 and hsp90 family in the rat cerebellum using immunocytochemistry with specific monoclonal antibodies. Our results showed that an intense immunostaining was evident, but was restricted in certain distinct cerebellar areas only, while no differences in the distribution of the two hsps were found. The strongest response was detected in the Purkinje neurons but deep cerebellar nuclei were also positive. In no case glial cells were found to be reactive for hsps despite their strong response for specific markers like glial fibrillary acid protein (astrocytes) and cyclic nucleotide phosphodiesterase (oligodendrocytes). These data indicate that both the hsp70 and hsp90 family have fundamental physiological functions in cerebellar neurons while they seem to play only a minor role in the metabolism of glial cells.