The invention relates to a method for determining the concentration of asymmetric dimethylarginine (ADMA) simultaneously with arginine and with symmetric dimethylarginine (SDMA) in biological samples by means of HPLC-MS-MS. The sample preparation consists exclusively of adding a solution of isotope-labeled internal standards (.sup.13C.sub.6-arginine and D.sub.6-ADMA) and of adding a mixture consisting of acetonitrile/propionic acid/trifluoroacetic acid for precipitating high-molecular proteins, the quantity and composition being realized in such a manner that a bringing of the sample composition in line with the composition of the mobile phase in the HPLC separation is achieved without requiring a derivitization or extraction of the analytes. The chromatographic separation of the analytes ensues on a silica normal phase HPLC column while using a mobile phase consisting of water/acetonitrile/propionic acid/trifluoroacetic acid, the volume ratio of water to acetonitrile ranging from 2 to 98 to 30 to 70, the volume percent of trifluoroacetic acid ranging from 0.01 to 0.5, the volume percent of propionic acid being 10 to 100 times higher than trifluoroacetic acid, and the optimized composition of the mobile phase consists of acetonitrile/propionic acid/trifluoroacetic acid with 10/90/1/0.025 volume percents. The detection and quantification ensue by means of tandem mass spectrometry with the following fragmentations being observed: 175.2 m/z.fwdarw.70.1 m/z for arginine, 181.2 m/z.fwdarw.74.1 m/z for .sup.13C.sub.6-arginine, 203.2 m/z.fwdarw.172.1 m/z for SDMA, 203.2 m/z.fwdarw.46.1 m/z for ADMA and 209.2 m/z.fwdarw.70.1 m/z D.sub.6-ADMA.