Using the slice culture system of 9-day-old rat cerebellum, effects of insulin on cell death of developing granule neurons were examined. Apoptotic cells were observed after 3 days culture by the in situ nick end labeling technique. Insulin deprivation induced apoptosis of granule neurons largely in the external granular layer, but scarcely in the internal granular layer. Proliferation of external granular layer neurons during the early culture period was not affected significantly by the insulin deprivation. These results suggest that insulin may prevent apoptosis of premigratory granule neurons during the development of the cerebellar cortex.