The mouse mutant Dystonia musculorum exhibits pathological changes in the magnocellular neurons of the red nucleus. The present study shows that allelic differences occur in the age of onset and severity of this pathology. The magnocellular neurons of the Jackson allele (dtJ) almost completely disappear prior to 4 weeks of age while some of these cells are retained in the adult of the Albany strain (dtAlb). However, acetylcholinesterase histochemistry suggests that the remaining rubral neurons in dtAlb are nonfunctional. This pathology may contribute to the severe locomotor disturbances seen in these animals.