The superior colliculi of Dutch-belted rabbits aged 1 to 30 days were prepared according to Nissl and Golgi techniques and examined in comparison to the neuronal populations in the adult. It was found that the laminar dimensions of the adult are attained at 15 to 20 days, presumably due to glial and dendritic enlargement. The largest neuron in the superficial gray, the vertical cell, was found to mature earliest and to display the least evidence of dendritic development postnatally. Its dendrites do elongate from 0 to 15 days, but there are no growth cones or other unusual dendritic configurations. The intrinsic neurons of the superficial gray, the piriform and stellate cells show more evidence of postnatal dendritic development. Particularly in the case of stellate cells, there are dendritic growth cones present at birth, and early in development spines are found on the soma and large dendrites. Only at 15 to 20 days do the dendritic arbors of these cells assume normal adult shapes and sizes. These and other observations suggest that there are two phases of postnatal cellular maturation in the upper layers of the superior colliculus, one involving the smaller "interneurons." This phased maturation is compared with the known sequence of physiological maturation as studied by the method of receptive-field mapping.