A quantitative assay for 4-hydroxybutyric acid was developed using D6-4-hydroxybutyric acid as an internal standard. 4-Hydroxybutyric acid was isolated by liquid chromatography and the amount quantified by selected ion monitoring, ammonia chemical ionization gas chromatography/mass spectrometry of the trimethylsilyl derivatives. The concentrations of 4-hydroxybutyric in control physiological fluids were: 2.64 +/- 3.46 mmol mol-1 creatinine in urine, 1.09 +/- 2.87 mumol l-1 in plasma, 0.98 +/- 1.17 mumol l-1 in cerebrospinal fluid and 1.28 +/- 0.47 mumol l-1 in amniotic fluid. The concentration of 4-hydroxybutyric acid in the amniotic fluid from a pregnancy at risk for 4-hydroxybutyric aciduria was 2.30 mumol l-1, indicating an unaffected fetus. The stable isotope dilution assay of 4-hydroxybutyric acid in physiological fluid samples is a rapid, sensitive and accurate method for quantification, as well as a valuable technique for the prenatal diagnosis of 4-hydroxybutyric aciduria.