We have examined the intracellular distribution of heat shock proteins HSP27 and HSP90 by means of specific antibodies and ultrastructural immunocytochemistry in the normal mouse testis as well as after heat shock. In the unstressed testis, these heat shock proteins are present in the cytoplasm and to a lesser extent in the nucleus throughout spermatogenesis. They do not show preferential association with any specific cytoplasmic structures and are absent from mitochondria. They disappear from the cell nucleus at the stage of elongating spermatids. After heat shock (42 degrees C), both HSP90 and HSP27 increase and partly relocate to the nucleus. Similarly to the localization in unstressed cells, they are mainly associated with perichromatin fibrils and the nucleolus. Moreover, a remarkable increase in the frequency of perichromatin fibrils in Sertoli cells, spermatogonia, and primary spermatocytes is also observed upon heat shock. Finally, a sharp increase in the labeling of HSPs in chromatoid bodies of round spermatids occurs following hyperthermic treatment. Interestingly, these two HSPs are localized on nuclear structures which are actively involved in RNA synthesis and processing, suggesting that they may have protective functions in these processes in a tissue which is particularly sensitive to heat stresses.