Quantification of 2-ketoglutaric acid in plasma and cerebrospinal fluid as its O-trimethylsilyl++-quinoxalinol derivative by gas chromatography chemical ionization mass spectrometry is described with benzoylformic acid as internal standard. This technique, with ammonia as reactant gas, only detects the protonated molecular ions. The recovery of 2-ketoglutarate from perchloric-deproteinized plasma is 99.7 +/- 1.2%. The normal value of 2-ketoglutarate in children is 8.6 +/- 2.6 mumol l-1 (mean +/- standard deviation) in plasma (n = 25) and 4.8 +/- 1.4 mumol l-1 in cerebrospinal fluid (n = 20). The plasma level of 2-ketoglutarate is correlated with urea concentration (r = 0.96; p less than 0.001) in healthy subjects and in patients with chronic renal insufficiency. Increased values are found in one case of pyruvate carboxylase deficiency, and inconstantly in diabetes; physiological variations are described during fasting and after an oral glucose load.