We analyzed the distribution of nonphosphorylated high-molecular-weight neurofilaments (NF-H) in the hippocampal formation of infant (3-week-old and 3-month-old) and adult (9-17-year-old) macaque monkeys in order to obtain neuroanatomical evidence of the maturity of these structures shortly after birth. We employed the monoclonal antibody SMI-32, a well-characterized antibody raised against nonphosphorylated NF-H, the expression of which is believed to reflect the maturation of certain neuronal populations. Patterns of SMI-32 immunoreactivity differed dramatically between infant and adult monkeys. In adults, nonphosphorylated NF-H expression was prominent in the CA3 and CA2 fields of the hippocampus, in the subiculum and in the entorhinal cortex. In infants, only the subiculum stained heavily for nonphosphorylated NF-H. These findings suggest that different subregions of the primate hippocampal formation mature at different times during development. The subiculum, the major source of efferent projections from the hippocampal formation toward subcortical structures, matures early during development. In contrast, the entorhinal cortex, the main interface of the hippocampal formation with the neocortex, matures relatively later. These findings have direct implications for the type of information processing that might be subserved by the primate hippocampal formation shortly after birth, as well as for the emergence of particular behavioral and memory processes during postnatal development.