The circling rat is an autosomal recessive mutant (homozygous ci2/ci2) characterized by lateralized rotational behavior, locomotor hyperactivity, ataxia, stereotypic head movements, and deafness. Previous neurochemical investigations showed that ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of circling. For further evaluation as to whether this striatal imbalance has functional consequences within basal ganglia structures, the spontaneous extracellular single unit activity of GABAergic neurons located in the striatum and, downstream to the dopaminergic nigrostriatal system, the substantia nigra pars reticulata (SNr) was recorded bilaterally in anesthetized ci2 rats. Heterozygous (ci2/+) littermates that display normal behavior, and rats from the background strain (LEW/Ztm) served as controls. No significant hemispheric imbalances in striatal discharge rate and firing pattern were evident in ci2 rats. Furthermore, there were no significant intergroup differences in striatal activity. However, the mean spontaneous discharge rate of SNr neurons was significantly increased in both brain sides, and there was a significant shift toward rhythmic burst-like firing in ci2 mutants. Again, no hemispheric differences were detected. The data substantiate previous findings of altered basal ganglia function in ci2 rats. The abnormal basal ganglia output activity, i.e. of the SNr, is likely to contribute to the complex behavioral disturbances seen in ci2 rats.