A single dose of actinomycin was applied to young Wistar albino rats in the critical phase of their cerebellar development. The morphological alterations of the cerebellar cortex were studied by means of light and electron microscopy on several postnatal days. The cell types of the cerebellar cortex reacted in different ways toward the noxious substance according to their stage of development. The acute alterations consisted of an edematous reaction of the neuronal and glial perikarya (light degeneration) and a shrinkage of the neurons (dark degeneration). A massive intercellular edema and a rarefaction of glia cells as well as the Purkinje cell fibres proved to be the long-term damage. This pattern of the alteration was discussed regarding the chemodifferentiation of the cells of the cerebellar cortex, the onset of cerebellar function on day 14, and the establishment of a neuroglial functional unit.