The effects of folic acid and vit. B12 on the toxicity of benzoic acid and on excretion of hippuric acid were studied. Benzoic acid at levels of 2 to 5% produced high mortality and the toxicity was not decreased by folic acid or serine supplementation. Detoxification of benzoic acid was not affected or only slightly reduced in folic acid deficiency. Further supplementation of the benzoic acid diets with DL-serine had no effect on hippuric acid excretion with or without folic acid. Vitamin B12 deficiency resulted in a greater reduction of hippuric acid excretion than did folic acid deficiency when the rats were given a test dose of benzoic acid by injection. The inclusion of benzoic acid in the diet decreased urinary excretion of FIGLU in folic acid-deficient rats. On the other hand, serine supplementation increased FIGLU excretion in the vitamin-deficient animals with or without added benzoic acid.