The morphology of the cerebellar cortex of the murine mutant "jolting" and its phenotypically normal littermate was studied. The general organisation of the cerebellum of normal mice was similar in all respects to that described for other vertebrate species. In "jolting" mice aged 4 months or more there was a marked loss of Purkinje cells and spheroids were present on Purkinje cell axons. An ultrastructural examination showed that the spheroids contained randomly orientated neurofilaments, degenerating mitochondria and autophagic vacuoles. The abnormalities in the cerebellum appeared confined to the Purkinje cells. Extracellular recordings were made of the electrical activity of the cerebellar cortex of anaesthetised mice. In the cerebellar cortex of "jolting" animals, Purkinje cells generated little spontaneous activity, but complex discharges resulting from climbing fibre inputs were indistinguishable from normal. The abnormal electrical activity of Purkinje cells in "jolting" mice could be seen in animals as young as 3 weeks. It was concluded that the "jolting" mouse suffers from a cerebellar ataxia.