Methyl mercury intoxication causes ataxia. Structural changes of cerebellar and peripheral nerve tissues have been described. However, it is still unclear whether the ataxia is of cerebellar or peripheral origin. To clarify this question further, the effects of methyl mercury intoxication on the numbers of granule and Purkinje cells and the volume of Purkinje cell perikarya have been evaluated with stereological methods. Rats were intoxicated with methyl mercury, at a dose of 2 mg/kg per day for 19 successive days, and the analysis was carried out 2.5 or 4.5 weeks later. The total numbers of cerebellar granule cells and Purkinje cells were estimated using an optical fractionator and the mean volume of the Purkinje cells was estimated by the vertical rotator technique. The volumes of the granular cell layer, the molecular layer and the white matter were estimated using the Cavalieri principle. The intoxicated animals developed hindlimb incoordination when held by the tail. Although pronounced axonal degeneration occurred in the peripheral nervous system, no changes were found in cerebellar cell numbers or cell sizes in either of the test groups. The absence of detectable light microscopic changes in the cerebellum indicates that the peripheral nervous system is affected prior to the cerebellum in rats intoxicated with organic mercury.