A major program in the mass spectrometry laboratory at NIEHS is the application of tandem mass spectrometric techniques (MS/MS) to the structure elucidation of compounds of interest in the environmental health sciences. The structure determination of these compounds is basic to understanding the interactions of compounds within the body, especially those due to altered metabolism and those arising through the interactions of xenobiotics and biomolecules. These techniques are important because samples of interest are often complex mixtures and because the ionization techniques applicable to these samples often provide little or no structural information. Our approach to the development of MS/MS techniques is twofold; structure elucidation and increasing the sensitivity of the technique. Current projects in the area of structure determination include: 1) glutathione, cysteine and N-acetylcysteine conjugates of xenobiotics, including identification of conjugates excreted from challenged animals; 2) determination of the location of substituents and double bonds in compounds within the arachidonic acid cascade under negative ion FAB/MS/MS conditions; 3) development of hybrid MS/MS techniques selected reaction monitoring the analysis of polychlorinated aromatic compounds such as dibenzofurans and dibenzodioxins; and 4) comparison of the utility of hybrid MS/MS vs. high energy MS/MS. A major effort in increasing MS/MS sensitivity has been in the combination of high flux/low level introductory systems such as OTLC and CZE. We have successfully lowered the MS/MS acquisition levels several orders of magnitude for a number of analyte types including peptides, protein digests, nucleotides, carcinogen-modified nucleosides, phospholipids and carbohydrates.