I . Experiments were conducted with young male broiler-type chicks to determine changes that may occur in serum aminotransferase activities and plasma free amino acid concentrations when chicks are given a purified diet containing levels of pyridoxine ranging from 1.1 to 7.1 2. Varying the dietary levels of pyridoxine had no significant effect on serum alanine aminotransferase (EC 2 . 6 . I . 2) activity whereas serum aspartate aminotransferase (EC 2.6. I . I ) activity decreased in chicks given diets containing less than 3 mg pyridoxine/kg diet.3. The effect of addition of pyridoxal phosphate to the serum (5opglo.z ml) before measuring the enzyme activity was also studied. I t was observed that a direct measure of the activity of serum aspartate aminotransferase is as sensitive an indicator of the pyridoxine nutritional status as the in vitro response to addition of pyridoxal phosphate.
4.Plasma free amino acid patterns were altered in the groups receiving inadcquatc levcls of pyridoxine ( 1 . 1 and 2.0 mg/kg). There were significant decreases in serine, taurine, a-amino butyric acid and 'asparagine -I-glutamine' concentrations, and increases in those of ornithine, glycine and cysteic acid.5 . The requirements of pyridoxine for body-weight gain, food consumption and food utilization in young growing chickens are less than for maximum serum aspartate aminotransferase Icvels. The increase in the ratio of g1ycine:serine in plasma appeared to be a consistent criterion of the pyridoxine-deficiency state in the young, growing chicken. mg/kg.