Post-translational methylation of arginine residues in proteins leads to generation of N(G)-monomethylarginine (MMA) and both symmetric and asymmetric dimethylarginine (SDMA and ADMA), that are released into the cytosol upon proteolysis. Both MMA and ADMA are inhibitors of nitric oxide synthase and especially elevated levels of ADMA are associated with endothelial dysfunction and cardiovascular disease. Plasma concentrations of ADMA and SDMA are very low, typically between 0.3 and 0.8 microM, making their quantification by HPLC an analytical challenge. Sample preparation usually involves a cleanup step by solid-phase extraction on cation-exchange columns followed by derivatization of amino acids into fluorescent adducts. Because ADMA and SDMA concentrations in healthy subjects show a very narrow distribution, with a between-subject variability of 13% for ADMA and 19% for SDMA, very low imprecision is an essential assay feature. Procedures for sample cleanup, derivatization, and chromatographic separation of arginine and its methylated analogs are the main topics of this review. In addition, important aspects of method validation, pre-analytical factors, and reference values are discussed.