Department ot Internal Medicme, U n i i w ~z t v of We used the single-pass-perfused rat liver model to study short-term regulation of endotoxin-inducible nitric oxide synthesis by following the release of nitrite and nitrate, the oxidation products of nitric oxide, into the effluent perfusate. In endotoxin-pretreated livers, the basal nitrite + nitrate release was 5.3 2 1.2 nmol.gm liver-'min-'. Nitrite and nitrate release was stimulated by L-arginine in a dose-dependent and saturable fashion. Maximal nitrite + nitrate release with 1 mmol/L L-arginine infused to the infiuent perfusate was 10.2 2 1.1 nmol . gm liver-' . min-', with a half-maximal effect at 53 pmol/L L-arginine. In the absence of molecular oxygen, nitric oxide synthesis was inhibited. Ammonium chloride infusion also stimulated nitrite and nitrate release to a maximal rate of 9.2 -C 0.8 nmol * g m liver-I . min-. I with half-maximal effects at 60 pmol/L ammonium chloride. Ammonium chloride-stimulated nitrite and nitrate release was abolished when urea synthesis was inhibited by bicarbonate-free liver perfusion. Citrulline and ornithine (200 pmol/L each) were without effect on nitrite and nitrate release. L-Nitroarginine methyl ester inhibited both, L-arginine-and ammonium chloride-induced nitrite and nitrate release. Stimulation of nitric oxide synthesis by L-arginine and ammonium chloride addition (1 mmol/L each) was accompanied by a threefold-to-fourfold increase of cyclic GMP release into the effluent perfusate. In livers of endotoxin-pretreated rats the urea production from L-arginine was higher than that in untreated livers, suggesting induction of an L-arginine transport system in hepatocytes by endotoxin. The regulation of hepatic nitric oxide production by physiological concentrations of L-arginine and ammonia in the portal vein may be of importance in cirrhosis. (HEPATOLOGY 1994; 19:641-647.) Nitric oxide (NO) is a mediator molecule that has been identified as the endothelium-derived relaxing factor involved in blood pressure regulation ( 1-31, In addition,