Inosine-5''-phosphate dehydrogenase catalyzes the 1st step in the conversion of inosine-5''-phosphate (IMP) to guanosine-5''-phosphate (GMP). This enzyme was isolated from rat liver to determine whether or not it was inhibited by thyroid hormones. Maximum inhibition was observed with 10-9 M of 3, 5, 3''-triiodo-L-thyronine (L-T3) but the inhibitory effect decreased as the concentration of the hormone increased. The inhibitory effect of L-thyroxine (L-T4) was also a peak at 10-7 M. When both hormones were incubated simultaneously with the enzyme (10-9 M of L-T3 with 10-7 M of L-T4), the inhibitory effect was greater than that due to either hormone alone. D-Thyroxine at 10-9 M or 10-7 Mhadno inhibitory effect, but at 10-5 M the reaction was stimulated. Certain other analogues, considered to be inactive in vivo, also had a stimulatory effect. Analogues considered to be active in vivo inhibited the reaction but no maximum was observed with these analogues up to 10-5 M. Inosine-5''-phosphate dehydrogenase of rat brain was not inhibited by either L-T3 or L-T4. The same enzyme from testes was inhibited slightly by L-T3 or L-T4. Thyroid hormones stimulate adenylosuccinate synthetase and inhibit IMP-dehydrogenase, which would result in a high cellular adenosine-5''-phosphate/guanosine-5''-phosphate (AMP/ GMP) ratio.