Changes in immunoreactive somatostatin (SOM) and cholecystokinin (CCK-8) levels in the cerebellum and cerebrum were investigated in three types of genetically - determined ataxic mutant mice: rolling mouse Nagoya (RMN), weaver, and Purkinje cell degeneration (PCD) mice. The cerebellar pathology in each of these types differes. The concentration of both SOM and CCK-8 (ng/mg weight) was significantly higher in the cerebellum and the cerebrum of the three types of ataxic mutant mice than in these regions in the respective controls. SOM and CCK-8 content (ng/organ) was significantly higher in PCD and RMN than in controls but this was not in the weaver mice. The possible involvement of both peptides in manifestations of ataxia is discussed.