The distribution of constitutively expressed mRNA encoding 2 isoforms of heat shock protein 90 (HSP90) was investigated in adult rat brain by in situ hybridization with 35S-labeled cRNA probes. HSP90 mRNA density was estimated by semiquantitative autoradiography and by microscopic analysis. The distribution of HSP90 transcripts was similar for the 2 probes. HSP90 mRNA was widespread but not ubiquitous, and labeling intensity varied among different regions. In particular, HSP90 mRNA was abundant in limbic system-related structures, such as the hippocampus, amygdala, mamillary body, piriform cortex, enthorhinal cortex, bed nucleus of the stria terminalis, medial habenular nucleus, and preoptic hypothalamic nuclei. The highest abundance of mRNA was detected in the Purkinje cell layer of the cerebellum. High abundance was also present in pineal body, choroid plexus, cerebellar granule cell layer, and cranial nerve nuclei. HSP90 mRNA was present in moderate abundance throughout the cortical gray mantle, in the caudate-putamen, most other thalamic and hypothalamic nuclei, the pontine and spinal gray matter, and the anterior lobe of the hypophysis. Weak hybridization was observed in white matter fiber tracts, glial cells, and leptomeninges, and none was observed in endothelial cells and the smooth muscle of the cerebral microvasculature. Our findings suggest that HSP90 gene products may serve constitutive neuron-specific functions in the unstressed brain.