Cell nuclei from developing rat cerebellum were isolated and the various types of nuclei were characterized and quantified. Nuclear pellets appeared to be both quantitatively and qualitatively representative of the entire cerebellum, and of sufficient purity to perform biochemical studies as well as morphological comparison with histological sections. Isolated nuclei were classified into 6 groups based on nuclear size and shape, heterochromatin aggregations, and nucleoplasmic density. The total population of cerebellar cells primarily consisted of two types of nuclei after day 10. One group of nuclei, resembling those of internal granule neurons or external germinal cells, contributed at least 70% of the total isolated cell nuclei from day 1 to day 90, whereas another nuclear group that was identified as dark oligodendrocytes constituted 8-9% of the total population on days 45 and 90. Nuclear DNA, RNA, and protein content of the cerebellum also were determined throughout postnatal development. DNA concentration markedly declined after day 15, while the RNA/DNA ratio increased until day 3 and remained constant to day 90. The nuclear protein/DNA ratio increased from birth to day 3, decreased to its lowest value on day 10, and increased to day 90. Utilizing DNA values, the total cell population as well as contributions of different cell types were calculated. At birth the cerebellum was estimated to contain 5.9 million cells, increasing to 94 million by day 21. By day 90, 107 million cells were present, of which 8.6 million oligodendrocytes and 93.6 million internal granule cells were estimated.