Previous results from our laboratory (Bejar et al. 1985) indicated that a single injection in mouse pups of the antimitotic/mutagenic agent methylazoxymethanol at postnatal day 5 typically produces hypogranular cerebella with no changes in foliation, in contrast to the severe alterations observed after the more usual injection on the day of birth. Here we report that injection of a higher dose (30 mg/kg) of methylazoxymethanol, always at postnatal day 5, leads to the additional presence of a ectopic cell layer in adult cerebellum. Immunostaining with several antibodies recognizing cell specific proteins ruled out the possibility that these ectopic cells were glial and electron microscopy indicated that they were morphologically mature granule cells. In the molecular layer of other cerebellar areas and apparently unrelated with granule cell ectopia, ectopic Golgi epithelial cells were observed. The reason for the presence of these ectopic cells of different type in the molecular layer was discussed in relation with analogous ectopias obtained by other means.