Postnatal ontogeny of dopamine-beta-hydroxylase (DBH)-like immunoreactive fibers in the rat cerebellar cortex has been studied by immunofluorescence on sagittal sections of the vermis. Except in the case of unusual thick highly fluorescent fibers noticed in the external granular layer during the first 2 weeks after birth, the arrival of DBH-containing fibers through the cerebellar cortex follows an inside-out gradient: at birth sparse fluorescent axons are observed in the album of the cerebellum. They invade the internal granular layer at postnatal day 2 (P2) and reach the Purkinje cell layer at P5. From P5 to P15, the density of fibers increases in this layer, which becomes the most innervated layer of the cerebellar cortex, as in the adult. When the external grains migrate, DBH-containing fibers extend progressively from depth to surface in the molecular layer and show an adult pattern at P30. The relationship between our results and other morphological and biochemical studies is discussed.