Amino acid incorporation was measured in a cell-free protein synthesis 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. Differential gene expression in the rat brain at the level of protein end-product was observed for a number of brain proteins, but the level of most brain proteins did not change with age. Differential gene expression also was observed for a number of brain mRNAs, but the majority did not change with age. The expression of the cellular homologues of three nuclear related oncogenes, v-fos, v-myc and v-myb, was examined in the rat brain. Sequences related to v-fos and v-myc but not to v-myb were detected. The expression of these sequences did not change with aging. In addition, the levels of B-actin mRNA did not change with age. Transcript levels of several proto-oncogenes were measured in brain and liver of rats by Northern blot hybridization analysis. An age-related increase in the level of c-myc mRNA in liver, but not brain, was observed. The levels of B-actin, c-sis, and c-src- related transcripts were age-invariant in both organs. The activity and relative levels of the pp60c-src protein in rat brain also were age-invariant.