The quantitative postnatal changes of the cerebella of 65 Wistar rats aged 2-120 days have been examined. The cerebellar volume increases in two phases: The first phase lasts from birth to the seventh postnatal week. The second phase begins ten weeks post partum and lasts for a longer period than the first phase. The cerebellar surface increases continuously from birth to the end of the seventh week. The volume of the external granular layer is maximal when the organ grows rapidly. The external granular layer has nearly disappeared 24 days after birth; the volume of the internal granular layer is maximal at this time. Later on, the volume and the width of the internal granular layer decrease. Myelinization of the cerebellar fibers and growth of the molecular layer run parallel to this decrease. The second late, but protracted growth of the cerebellum, ten weeks after birth, is due to an increase of the molecular and medullary layer. These findings are in good accord with histological, histochemical, and ultrastructural observations of other authors.