In clinical and forensic toxicology, doping control, and therapeutic drug monitoring, specific and sensitive detection and precise quantification of xenobiotics in biosamples are great challenges. Today, mass spectrometry techniques, coupled with gas chromatography or liquid chromatography, are the most powerful methods in analytic toxicology. The pros and cons of electron ionization (EI) and negative ion chemical ionization (NICI) gas chromatography-mass spectrometry (GC-MS) and of atmospheric pressure chemical ionization liquid chromatography-mass spectrometry (APCI-LC-MS) are described for determination of the low-dosed benzodiazepine flunitrazepam and its 7-amino and its nor-metabolite in plasma. In addition, application of NICI-GC-MS is described for sensitive chiral determination of amphetamine derivatives in plasma and application of APCI-LC-MS for screening, library-assisted identification, and validated quantification of oral antidiabetics and for validated quantification of the neuroleptic risperidone and its 9-hydroxy metabolite. These examples show that NICI-GC-MS and LC-MS are powerful tools for determination of low-dosed and/or rather polar drugs or poisons, thus becoming indispensable supplements to classic EI-GC-MS in clinical and forensic toxicology as well as in doping control.