The distribution of the calcium-binding proteins calbindin D-28K and parvalbumin was examined in newborn and adult superior colliculus of cat and rhesus monkey using immunohistochemical techniques. In adult animals of both species, calbindin-immunoreactive neurons had a three-tiered arrangement: one band was present in the upper aspects of the superficial laminae, a second in the intermediate laminae, and a third in the deep laminae. The intermediate tier was less obvious in the monkey, whereas the deep tier was less pronounced in the cat. Parvalbumin-immunoreactive neurons had a complementary distribution to calbindin-immunoreactive neurons within these laminae in both species, although the segregation of calbindin immunoreactivity and parvalbumin immunoreactivity in the superficial laminae was not as precise in the monkey as it was in the cat. At birth, calbindin immunoreactivity in the newborns of both species was remarkably mature, with its three-tiered distribution clearly evident. By contrast, parvalbumin immunoreactivity was distinctly different in the newborn cat than in the newborn monkey: whereas parvalbumin immunoreactivity in the newborn monkey was already very similar to its adult-like pattern, the pattern in the newborn cat was quite immature. The superficial laminae of the newborn cat were virtually devoid of parvalbumin immunoreactivity, and, although the intermediate laminae displayed robust parvalbumin-immunoreactive neuropil, comparatively fewer parvalbumin-immunoreactive neurons were observed. Conspicuously few in number were the large multipolar neurons in the intermediate laminae, which give rise to the descending efferents to the brainstem. However, parvalbumin-immunoreactive neurons were present within the deep laminae, suggesting a ventral-to-dorsal maturational gradient in parvalbumin expression that parallels the ventral-to-dorsal gradient of neurogenesis. The differences in parvalbumin immunoreactivity observed between these two species at parturition are consistent with the advanced visual and visuomotor capabilities of the newborn monkey and the absence of visually related behaviors in the newborn cat.