Methylarginines are endogenous inhibitors of nitric oxide synthase (NOS) and have been implicated in the regulation of the nitric oxide pathway in health and disease. Cellular concentrations of free methylarginines are determined in part by the activity of dimethylarginine dimethylaminohydrolase (DDAH). There are two isoforms of DDAH which have distinct tissue distributions with some relationship to NOS isoforms. We have determined the intracellular localization of both DDAH isoforms by overexpression of epitope-tagged DDAH in an immortalized endothelial cell line. Immunofluorescence confocal microscopy and immunoblotting indicate that both isoforms are predominantly cytosolic with no specific association with organelles or the plasma membrane. These data suggest that the key role for DDAH may be to ensure that under normal conditions the levels of methylarginines are kept low throughout the whole cell.