Amino acid incorporation was measured in a cell-free protein systhesis system derived from the brains of male Fischer-344 rats of different ages. This system has the capacity to initiate protein synthesis in vitro. There was no significant correlation between protein synthesis and age. A procedure for isolating neurofilament proteins from the rat central nervous system was developed. Analysis of neurofilament proteins by two dimensional gel electrophoresis revealed the three neurofilament proteins of 210K, 160K, and 68K daltons molecular weight. This technique revealed heterogeneity in the neurofilament protein pattern, especially in the 68K dalton polypeptide. Analysis of gene expression in 11 individual regions of the rat brain at the level of protein end-product was conducted using two dimensional gel electrophoresis in conjunction with a sensitive silver stain protein detection procedure. Differential gene expression was observed for a number of brain proteins. Regulation at the level of protein end-product was not observed for a significant number of brain proteins.