ExtractHuman infants with eytomegalovirus and herpes simplex are presently treated with floxuridine (5-FUDR), a DNA synthesis blocking agent. Since in the cerebellum of the newborn infant neuron formation continues for some time after birth, the question arises whether 5-FUDR might cause permanent damage to the cerebellum. In our experiment 21 2-day-old mice were treated by three injections of 5-FUDR (50 mg/kg body weight per injection), and particular attention was given to damage and subsequent repair of the external granular layer, the site of postnatal neuron formation. In the anterior lobes of the vermis most proliferating external granular cells died and repair was minimal. Purkinje cells were dispersed and few if any basket cells were formed. In the posterior lobes, including the uvula, considerable repair of the external granular layer was noted. The final cerebellar architecture, however, was never normal, and heterotopic granule cell nests were found in the molecular layer. In the intermediary lobes repair occurred, but Purkinje cells were found throughout the granular layer. Since the human cerebellum at birth is characterized by a thick external granular layer, it is not impossible that 5-FUDR may also cause disturbances in the human cerebellar architecture and function.