We have examined the number of "blobs" (cytochrome oxidase-positive cortical modules) in the primary visual cortex (area 17) of infant and adult rhesus monkeys. The density of these iterated circuits--about five per mm2--was not significantly different in three newborn and three mature animals. Measurement of the surface of area 17 in serial sections, however, showed that the total area occupied by the primary visual cortex increases by about 50% during maturation. Based on these measurements, the number of blobs in this species is about 8000 at birth and about 12,000 in maturity. Evidently, these complex functional units are added gradually to the developing primate brain over a period that extends into postnatal life.