shaker Mutant rats were first identified by their abnormal motor behaviors and degeneration of cerebellar Purkinje cells and brainstem inferior olivary neurons. After 6 generations of inbreeding 77% of shaker rat mutants are mildly ataxic (identified as mild shaker mutants) and 23% are ataxic and exhibit a whole body tremor (strong shaker mutants) by 3 months of age. This study of shaker mutants from birth to 3 months of age was designed to: (1) compare the somatic and motor development of shaker mutants with age matched normal rats; (2) identify the temporal onset of motor deficits; and (3) correlate qualitative differences in Purkinje cell degeneration between 3-month-old mild and strong shaker rat mutants. Shaker mutant rats consistently weighed less than age-matched control animals. Analysis of motor-development using the hindlimb splay test demonstrated the distance between hindpaws was significantly greater in shaker mutant rats than in controls starting at 42 postnatal days (PND) of age. Hindlimb stride width was greater for shaker than control rats at 42 PNDs. However, after 42 PNDS shaker mutant average hindlimb width was narrower than controls. Forelimb stride width was consistently narrower in shaker mutants than in normal rats. Hindlimb placement was impaired in shaker rat mutants after 15 PND. Forelimb placement, cliff avoidance and surface righting were only transiently impaired in shaker mutants. Mid-air righting, performance of a geotaxic response, and climbing and jumping postural reactions were similar in shaker and normal rats. The spatial extent of Purkinje cell survival/degeneration correlated with differences in abnormal motor activity seen in 3-month-old mild and strong shaker mutants. In mild shaker rat mutants, Purkinje cells appeared to have degenerated randomly throughout the cortex. In strong shaker mutants most Purkinje cells in the anterior lobe had degenerated. In the posterior lobe Purkinje cell degeneration appeared to be numerically significant, but many surviving cells were present. Although Purkinje cell loss was not numerically quantified in this study, a strong association between the extent and type of spatial loss of Purkinje cells, and the severity of clinical signs, appears to exist.