l-Arginine metabolism is important in the maintenance of airway tone. Shift of metabolism from the nitric oxide synthase to arginase pathways contributes to the increased airway responsiveness in asthma. We tested the hypothesis that systemic levels of l-arginine metabolites are biomarkers reflective of airway dysfunction. We used a mouse model of acute allergic airway inflammation to OVA that manifests with significant airway hyperresponsiveness to methacholine. To determine tissue arginase activity in vivo, the isotopic enrichment of an infused l-arginine stable isotope and its product amino acid l-ornithine were measured in lung and airway homogenates using liquid chromatography-tandem mass spectrometry. Tissue and plasma concentrations of other l-arginine metabolites, including l-citrulline and symmetric and asymmetric dimethylarginine, were measured and correlated with lung arginase activity and methacholine responsiveness of the airways. The effectiveness of intratracheal instillation of an arginase inhibitor (boronoethylcysteine) on pulmonary arginase activity and circulating concentrations of l-arginine metabolites was also studied. We demonstrate that 1) plasma indexes of l-arginine bioavailability and impairment of nitric oxide synthase function correlate with airway responsiveness to methacholine; 2) plasma levels of l-ornithine predict in vivo pulmonary arginase activity and airway function; and 3) acute arginase inhibition reduces in vivo pulmonary arginase activity to control levels and normalizes plasma l-ornithine, but not l-arginine, bioavailability in this model. We conclude that plasma l-ornithine may be useful as a systemic biomarker to predict responses to therapeutic interventions targeting airway arginase in asthma.