N-Nitrosodimethylamine (NDMA), a member of a group of probable human carcinogens, has been detected as a disinfection byproduct (DBP) in drinking water supplies in Canada and the United States. To comprehensively investigate the occurrence of possible nitrosamines in drinking water supplies, a liquid chromatography-tandem mass spectrometry technique was developed to detect both thermally stable and unstable nitrosamines. This technique consisted of solid-phase extraction (SPE), liquid chromatography (LC) separation, and tandem quadrupole linear ion trap mass spectrometry (MS/MS) detection. It enabled the determination of sub-ng/L levels of nine nitrosamines. Isotope-labeled N-nitrosodimethylamine-d6 (NDMA-d6) was used as the surrogate standard for determining recovery, and N-nitrosodi-n-propylamine-dl4 (NDPA-dl4) was used as the internal standard for quantification. The method detection limits were estimated to be 0.1-10.6 ng/L, and the average recoveries were 41-111% for the nine nitrosamines; of these, NDMA, N-nitrosopyrrolidine (NPyr), N-nitrosopiperidine (NPip), and N-nitrosodiphenylamine (NDPhA) were identified and quantified in drinking water samples collected from four locations within the same distribution system. In general, the concentrations of these four nitrosamines in this distribution system increased with increasing distance from the water treatment plant, indicating that the amount of formation was greater than the amount of decomposition within this time frame. The identification of NPip and NDPhA in drinking water systems and the distribution profiles of these nitrosamines have not been reported previously. These nitrosamines are toxic, and their presence as DBPs in drinking water may have toxicological relevance.