In rats, fed a folic acid-deficient synthetic diet containing sulfaguanidine, growth was retarded, and decreases in the red cell count and granulocyte count and a relative increase of hypersegmented neutrophils were noted. The administration of methotrexate (2 μg per day per rat, intramuscularly) for 3 days beginning on the 18th day of the experimental diet exaggerated these hematological changes. A single injection of tetrahydrofolic acid, 5-methyl-tetrahydrofolic acid or 5.10-methenyltetrahydrofolic acid (500lμg per rat) on the 21th day of the experiment resulted in increases in the red cell count and the granulocyte count, also in the normalization of distribution of nuclear segments in neutrophils on the 2nd day. Of all the compounds tested, tetrahydrofolic acid had the most potent effect. (Received April 22,1972)