A GC-MS method is reported for the determination of phytic acid based on purification by anion-exchange chromatography, enzymatic hydrolysis of phytic acid to myo-inositol and derivation to trimethylsilyl derivative, with scyllo-inositol as an internal standard. Analytical features of the method are: limit of detection 9 microg l(-1) phytic acid, linear working range 18-500 microg l(-1) phytic acid, and coefficient of variation 1.9%. The method has been successfully applied to a variety of biological samples: various rat organs (kidney, liver, brain and bone), human plasma and urine and kidney stones. A comparative study of sample treatments, including deproteization, lipid extraction and the presence of a chelator, is also reported. Phytic acid amounts found in rat organs ranged from 1.07 g kg(-1) for bone to 32.0 g kg(-1) for brain. Phytic acid in human plasma was of the order of 0.14 mg l(-1). In kidney stones, phytic acid was found in calcium containing stones.