Elevated levels of serum uric acid (UA) are commonly associated with primary pulmonary hypertension but have generally not been thought to have any causal role. Recent experimental studies, however, have suggested that UA may affect various vasoactive mediators. We therefore tested the hypothesis that UA might alter nitric oxide (NO) levels in pulmonary arterial endothelial cells (PAEC). In isolated porcine pulmonary artery segments (PAS), UA (7.5 mg/dl) inhibits acetylcholine-induced vasodilation. The incubation of PAEC with UA caused a dose-dependent decrease in NO and cGMP production stimulated by bradykinin or Ca²⁺-ionophore A23187. We explored cellular mechanisms by which UA might cause reduced NO production focusing on the effects of UA on the L-arginine/eNOS and L-arginine/arginase pathways. Incubation of PAEC with different concentrations of UA (2.5 - 15 mg/dl) for 24 h did not affect [3H]-L-arginine uptake or activity/expression of eNOS. However, PAEC incubated with UA (7.5 mg/dl; 24 h) released more urea in culture media than control PAEC suggesting that arginase activation might be involved in the UA effect. Kinetic analysis of arginase activity in PAEC lysates and rat liver and kidney homogenates demonstrated that UA activated arginase by increasing its affinity for L-arginine. An inhibitor of arginase (S)-(2-boronoethyl)-L-cysteine prevented UA-induced reduction of A23187-stimulated cGMP production by PAEC and abolished UA-induced inhibition of acetylcholine-stimulated vasodilation in PAS. We conclude that UA-induced arginase activation is a potential mechanism for reduction of NO production in PAEC.