The protein synthetic rate in the yeast S. cerevisiae, measured by the incorporation of radioactive amino acids per unit amount of proteins, decreased linearly with age reaching 50% of the rate of 2nd generation cells (young cells) in 20th generation cells (old cells), whereas the RNA content of the old cells was increased three times. Using a cell-free system for poly(U)-directed poly-phenylalanine synthesis, the activity of run-off ribosomes from old cells was shown to be about 40% less than the activity of ribosomes from young cells and the polysome level in old cells was much decreased compared to that in young cells. However, as protein content was increased twice in 20 generations, the cell is considered to maintain a constant level of protein synthesis during the process of aging compensating the decrease in the activity of ribosomes. Thus, it is likely that the decrease in the synthesis of certain proteins whose requirement was raised by the increase in cell volume, which is twice the increase in protein content, causes prolongation of the unbudded phase in old cells.