The in vivo effects of triiodothyronine on the synthesis of hepatic purine nucleotides was investigated in sulfaguanidine-fed hypothyroid rats. To prevent the rapid destruction of nucleotides caused by anoxia and ischemia an apparatus was designed to maintain ether anesthesia while providing normal levels of oxygen. Liver samples were obtained from anesthetized rats by freezing in situ. Administration of 15 µg of triiodothyronine to hypothyroid rats weighing 110-120 g resulted in stimulation of the incorporation of [1-¹⁴C]glycine into the total soluble adenine and guanine nucleotide pools as early as l hour following injection. The increase in the specific activity of adenine was greater than that of guanine for up to 4 hours. No increases were detected in the levels of total adenine or guanine. Hormone treatment stimulated the rate of incorporation of glycine into AMP and NAD⁺ within 1.5 hours. The magnitude of the observed increases in specific activity of AMP and NAD⁺ were dependent upon the relative timing of administration of hormone and glycine. The greatest increase in specific activity was observed in NAD⁺. The effect of the hormone on the synthesis of GMP was variable and dependent upon relative timing of administration of hormone and glycine. At the shortest times studied there was no apparent effect of the hormone on the specific activity of GMP. At longer times of hormone treatment a moderate decrease in specific activity was observed in the GMP. Treatment of rats with actinomycin D prior to hormone injection did not prevent the stimulation of synthesis due to triiodothyronine. Actinomycin D alone stimulated the incorporation of the precursor into the soluble nucleotides and magnified the effects of the hormone in increasing the specific activities of AMP and NAD⁺. / Master of Science