A potent, selective, competitive, and high affinity inhibitor of arginase. Shown to inhibit arginase from rat liver (IC50 = 2 μM) and mouse macrophages (IC50 = 50 μM). Shown to specifically interact with the manganese-cluster of the enzyme active site. Does not function as a substrate or as an inhibitor for NOS. Hence, nor-NOHA may be a useful tool to study the interplays between arginase and NOS.