Asymmetric dimethyl arginine (ADMA) is an endogenously produced nitric oxide synthase (NOS) inhibitor. L‐arginine can be metabolized by NOS and arginase, and arginase is the first step in polyamine production necessary for cellular proliferation. We tested the hypothesis that ADMA would inhibit NOS but not arginase activity, and that this pattern of inhibition would result in greater L‐arginine bioavailability to arginase and thereby increase viable cell number. Bovine arginase was used in in vitro activity assays with various concentrations of substrate (L‐arginine, ADMA, NG‐monomethyl‐L‐arginine (L‐NMMA) and NG‐nitro‐L‐arginine methyl ester (L‐NAME)). Only L‐arginine resulted in measurable urea production (Km = 6.9 ± 0.8 mM; Vmax = 6.6 ± 0.3 μmol/mg protein/min). We then incubated bovine arginase with increasing concentrations of ADMA, L‐NMMA, and L‐NAME in the presence of 1 mM L‐arginine, and found no effect of any of the tested compounds on arginase activity. Using bovine pulmonary arterial endothelial cells (bPAEC) we determined the effects of ADMA on NO and urea production, and found a significantly lower NO production and greater urea production (p<0.003) with ADMA, without changes in arginase protein levels. Additionally, ADMA treatment resulted in ~30% greater number of viable cells after 48 hours than in control bPAEC. These results demonstrate that ADMA is neither a substrate nor an inhibitor of arginase activity, and that in bPAEC ADMA inhibits NO production and enhances urea production leading to more viable cells. These results may have pathophysiological implications in disorders associated with higher ADMA levels such as pulmonary hypertension. This article is protected by copyright. All rights reserved.