[en] For quantitative determination of stable iosoptically labelled metabolic substrates in patients who had been fed deuterium labelled amino acids (50 mg per kg body wt.), the appearance and level of deuterated substrates and metabolites in the body fluids could be rapidly followed using direct mass spectrometry (DMS) without chromatographic separation in a time course tests. The per cent of deuteration could be calculated from (peak height of labelled) X 100 / peak height of (labelled + unlabelled) without the use of an internal standard. To determine the amino acid concentration of both the labelled and the unlablled species present in the sample, the addition of the same amino acid but differently labelled is required. If a differently labelled amino acid is not available for use as an internal standard, it is still possible to determine the concentration of the labelled amino acid. The technique is rapid and specific and the method is useful for the diagnosis and study of metastable defects. (Auth.)