Asymmetric N-omega,N-omega-dimethyl-L-arginine (ADMA) is an endogenously produced inhibitor of human nitric oxide synthase and an emerging biomarker for cardiovascular disease. Concentrations of ADMA are controlled by two isoforms of its catabolic enzyme dimethylarginine dimethylaminohydrolase (DDAH), the dysregulation of which has been studied as a mediating factor for endothelial dysfunction. A two-part, click-chemistry mediated activity-based probe, N-but-3-ynyl-2-chloroacetamidine, is shown to label myc-tagged DDAH-1 expressed in HEK 293T cells, but not an inactive mutant or inhibited enzyme. A two-color Western blotting technique is used to determine the in vivo IC50 value for a reversible inhibitor of DDAH-1, N-5-(1-iminopropyt)-L-ornithine, indicating this compound's bioavaitability and its competition for binding to the active site. This probe provides a novel tool for the analysis of DDAH-1 activity in normal and pathophysiological states and should allow more meaningful studies of the etiology of endothetial dysfunction.