Neurofilament (NF) proteins are expressed in most mature neurons in the central nervous system. Although they play a crucial role in neuronal growth, organization, shape, and plasticity, their expression pattern and cellular distribution in the developing hippocampus remain unknown. In the present study, we have used Western blotting and immunocytochemistry to study the low- (NF-L), medium- (NF-M), and high- (NF-H) molecular-weight NF proteins; phosphorylated epitopes of NF-M and NF-H; and a nonphosphorylated epitope of NF-H in the early postnatal (through P1-P21) development of the rat hippocampus. During the first postnatal week, NF-M was the most abundantly expressed NF, followed by NF-L, whereas the expression of NF-H was very low. Through P7-P14, the expression of NF-H increased dramatically and later began to plateau, as also occurred in the expression of NF-M and NF-L. At P1, no NF-M immunopositive cell bodies were detected, but cell processes in the CA1-CA3 fields were faintly immunopositive for NF-M and for the phosphorylated epitopes of NF-M and NF-H. At P7, CA3 pyramidal neurons were strongly immunopositive for NF-L and NF-H, but not for NF-M. The axons of granule cells, the mossy fibers (MFs), were NF-L and NF-M positive through P7-P21 but were NF-H immunonegative at all ages. Although they stained strongly for the phosphorylated NF-M and NF-H at P7, the staining intensity sharply decreased at P14 and remained so at P21. The cell bodies of CA1 pyramidal neurons and granule cells remained immunonegative against all five antibodies in all age groups. Our results show a different time course in the expression and differential cell type and cellular localization of the NF proteins in the developing hippocampus. These developmental changes could be of importance in determining the reactivity of hippocampal neurons in pathological conditions in the immature hippocampus.