In the liver prostaglandins have been shown to be potent regulators of portal blood flow, carbohydrate metabolism and bile secretion. It is not known whether these effects represent a direct action of prostaglandins, and it has been suggested that nitric oxide (NO) might be a critical mediator for prostaglandin induced hepatic events. We have studied whether nitric oxide formation or inhibition alters the action of prostaglandin F 2α (PG F 2α) in a single-pass liver perfusion model. The liver of untreated rats (constitutive NO-synthase) or after pretreatment with endotoxin (inducible form of NO-synthase) was perfused at a constant pressure via the portal vein. Effluate were collected in 1-min intervals and bile in 5-min intervals. In both groups the addition of PG F 2α (10 μM) to the perfusate for 5 min resulted in a significant increase of glucose and lactate production, and in a significant decrease in portal blood flow (−0.56 ± 0.04 ml/g per min), in bile flow (−60.7%) and in bile acid release (−60.6%). Inhibition of NO synthase by adding N G - monomethyl- l-arginine ( l-NMMA, 100 μM) to the perfusate did not affect any of the alterations induced by PG F 2α. Substitution of the endogenous substrate for the NO synthase l-arginine (500 μM) in the perfusate completely prevented the hemodynamic alterations induced by PG F 2α in endotoxin pretreated livers and limited the flow reduction (0.15 ± 0.04 ml/g per min) in the untreated group. The substitution of l-arginine in the perfusate of endotoxin pretreated livers raised nitrite (from 1.5 ± 0.3 to 3.6 ± 0.7 nmol/g per min) and urea release (from 65 ± 25 to 294 ± 68 nmol/g per min), but had no effect on any of the other metabolic parameters and bile secretion. We conclude that PG F 2α increases glucose and lactate production in the perfused rat liver and decreases portal flow and bile secretion. The metabolic effects induced by PG F 2α appear to be independent of NO mediation and hemodynamic alterations. Portal flow alone can be influenced by endogenous NO formation.