Addition of oxidized linoleate to the diet of rats resulted in a decreased rate of weight gain, an increased calorie requirement for each gram of weight gained, an impairment of fat absorption and an increase in the weight and length of the small intestine. The white cell count, blood haemoglobin levels and the concentration of folic acid in serum and urine were found to be reduced. Of the two metabolically active derivatives of folic acid examined in urine specimens, folinic acid levels showed a significant reduction but not those of N-10-formyltetrahydrofolic acid. Administration of folic acid to these animals resulted in a reticulocyte response and return of the white cell count to normal values.