Neurofilament protein (NFP) consists of three subunits: NF200, NF150 and NF68. Several studies on expression of NFP in developing brain have shown that NF200 appears later than NF150 and NF68. However, there are some reports on simultaneous appearance of these subunits in development. The present study is an attempt to resolve this controversy. Rat cerebellum was chosen as most of its development takes place during the first three weeks of postnatal period. Cytoskeletal and NFP preparations from newborn (P0), postnatal day 8 (P8), P15, P21, P30 and adult (3 months) rat cerebella were subjected to electrophoresis on 7.5% SDS-PAGE. All the NFP subunits were present from P0 onwards and there was an increase in NFP content and glial fibrillary acidic protein (GFAP) with age as revealed by the densitometric scanning. Immunoblots of NFP preparations confirmed the presence of NF200 in the early postnatal cerebellum. In vivo phosphorylation studies indicated the presence of phosphorylated NF subunits from P8 onwards, which was confirmed by staining in immunoblots by SMI31. Immunohistochemical studies on Bouin's fixed tissues revealed that in P0 cerebella, the deeper neurones (soma and processes) expressed all the NFP subunits while from P8 onwards they were negative for NF200. Similarly, Purkinje cells (soma) expressed transiently NF200 subunits on P8 and ceased to express them from P15 onwards. The white matter was immunopositive for NF200 and NF150 on P0 and the intensity of staining increased progressively. Astrocytes expressing GFAP were seen in cerebellar white matter from P8 onwards and the staining in radial glia could be detected from P15 onwards.(ABSTRACT TRUNCATED AT 250 WORDS)