Expression of corticotropin-releasing factor (CRF) mRNA was examined in the inferior olivary nucleus (ION) of an ataxic mutant, rolling mouse Nagoya (RMN) by semi-quantitative in situ hybridization. The most marked difference in the level of CRF mRNA signals between RMN and non-ataxic littermates (control mice) was observed in the beta-subnucleus and ventrolateral protrusion of the ION. The level of signals in these subnuclei was about twofold higher in RMN than in the controls. Signal levels in the dorsal nucleus, principal nucleus and subnucleus A were slightly but significantly higher in RMN than in the controls. In the other subnuclei, there were no differences in signal level between RMN and controls. These results suggest a region-related over-expression of CRF mRNA in the ION of RMN. This may be responsible for the increased sensitivity of some Purkinje cells to glutamate, resulting in ataxic symptoms of RMN.