Of the hyphenated techniques used for metabolic profiling of cell and tissue extracts, GC/MS is in some ways advantageous as it allows the simultaneous fingerprinting of chemically very different metabolites, and the electron impact mass spectra recorded in many cases lead to unambiguous identification of the compounds. However, prior to chromatography, the hydrophilic substances of the cell extracts have to be converted to vaporizable derivatives, the mass spectra of which often are not known or not listed in the available spectral libraries, even if they are derived from simple biochemicals. Thus, numerous chromatographic peaks remain as yet unidentified. Attempts to identify these peaks afford the acquisition of more data on these compounds. The value of in vivo labeling of metabolites with (13)C and (15)N for this purpose is described.