The effect of feeding thyroid powder on folic acid metabolism in rats was studied by feeding folic acid at levels of 0, 0.5 and 10 ppm in the absence and presence of 3 g thyroid powder/kg of diet. Thyroid feeding did not increase the folic acid requirements for growth but did increase the excretion of formiminoglutamic acid at folic acid levels of 0 and 0.5 ppm. The increased excretion of formiminoglutamic acid by thyroid feeding at suboptimal levels of folic acid was prevented by feeding a high level of folic acid (10 ppm).