Thyroxine was observed to stimulate the incorporation of radio labeled amino acids into protein of cell-free Liver systems from treated normal and hypothyroid rats. However, the in vitro addition of the hormone only had a stimulatory effect, though quite erratic in magnitude, on the normal system. This difference in response has not been explained. The hypothyroid rat was also characterized by lowered liver ribonucleic acid levels. It WEB found that thyroxine pretreatment resulted in increased ribonucleic acid levels only in the hypothyroid rat with the greater increase occurring in microsomal ribonucleic acid. No change was observed in the ribonucleic acid base ratios of the hypothyroid rat when compared to those of the normal. However, thyroxine pretreatment cf the hypothyroid rat did alter the rate of incorporation of radio phosphorus into the various nucleotides, although there was little difference in the total amount of incorporated label. / Ph. D.