Abstract: Oxidative cleavage of tartaric acid by means of periodic acid gave glyoxylic acid quantitatively, the latter was then condensed with p-nitrophenylhydrazine. The p-nitro-phenylhydrazone thus formed was purified by some extraction procedures and determined colorimetrically after treatment with sodium hydroxide. Quantities as small as 5 μg could be estimated by means of an electrophotometer. The average relative mean deviation was 4± 3%. Recoveries were 106±15% for urine and 74±6% for serum. The absorption spectrum of glyoxylic acid p-nitrophenylhydrazone after treatment with sodium hydroxide was determined. As the spectrum did not change even in the presence of antimony in quantities as much as 20 times the molar concentration of tartaric acid, this method would be suitable especially for the studies on the metabolism of the tartaric acid portion of potassium antimony tartrate.