In the cerebellum external germinal layer (EGL) regeneration was maximal 10 days following methylazoxymethanol-induced destruction of the postnatal mouse EGL. Similar patterns of EGL repair and delayed disappearance were observed in both anterior and posterior cerebellar lobes. Final reduction in granule cell number to 30-50% of control exceeded the reduction in basket and stellate cells. Morphological changes in GABAergic cell populations were paralleled by alterations in glutamic acid decarboxylase activity. These studies suggest that the regenerating EGL may be similar to the normal EGL with respect to the emerging cell types, but that granule cells which develop later are less completely represented.