The effect of thyroid hormones on purine biosynthesis was studied in vitro. With 100,000 x g supernatant fraction of rat liver homogenate, the results suggested that thyroid hormones stimulated total purine synthesis from labeled glycine-1-¹⁴C. Further studies indicated that these hormones stimulated AMP synthesis but inhibited GMP synthesis. The stimulatory effect on AMP synthesis was found to be due to the stimulation of adenylosuccinate synthetase. Adenylosuccinate synthetase was isolated and purified from rat liver. The maximum stimulatory effect of these hormones occurred with 2.5 x 10⁻⁵ M thyroxine (T₄) and 2.5 x 10⁻⁹ M triiodo-L-thyronine (T₃). A slight increase or decrease in concentration of these hormones caused a drastic decrease in their stimulatory effect. Some analogues of T₄ were also studied and results of such experiments agreed qualitatively with their effects in vivo. Those which are physiologically active are capable of stimulating this enzyme and those which are physiologically inactive had little or no effect on this enzyme. Based on these results, a hypothesis, that thyroid hormones regulate the levels of AMP and GMP synthesis, was proposed. The significance of this preferential stimulatory effect on AMP synthesis was discussed.Ph. D.