Northern blot and in situ hybridization were employed to investigate regional and cell type differences in the expression of hsp90 mRNA species in control and hyperthermic rabbit brain. Riboprobes specific to hsp90 alpha and beta mRNA species were utilized in time-course Northern blot studies on cerebral hemispheres and the cerebellum. Following hyperthermia, levels of hsp90 alpha and beta mRNA were elevated in both brain regions; however, the magnitude of induction was more robust in the cerebellum than in cerebral hemispheres. The pattern of expression of hsp90 genes in rabbit brain was analyzed by in situ hybridization. These studies revealed that hsp90 genes are preferentially expressed in neuronal cell populations in the unstressed mammalian brain. The distribution of hsp90 alpha and beta mRNA was similar, though the signal for the latter was stronger. Following hyperthermia, changes were not detected in the pattern of hsp90 beta mRNA expression in the hippocampus. In the cerebellum, a rapid induction of hsp90 beta mRNA was apparent in the neuron-enriched granule cell layer, followed by a delayed accumulation in Purkinje neurons. Unlike hsp70, induction of hsp90 was not detected in glial cells of hyperthermic rabbit brain. The localization of hsp90 to neurons suggests that this heat shock protein plays an important role in neuronal function.