Abstract Nicotinamide N-methyltransferase (NNMT), which converts nicotinamide (NA) to 1-methylnicotinamide (MNA), is up-regulated in the cirrhotic liver. Because MNA displays PGI2-dependent anti-inflammatory effects, the up-regulation of NNMT may play a regulatory role in liver inflammation. In the present work, we analyzed changes in NNMT activity in the liver and concomitant changes in the concentration of endogenous MNA in plasma in T-cell dependent hepatitis induced by concanavalin A (ConA) in BALB/c mice. Furthermore, we tested whether exogenous MNA possessed a protective effect against ConA-induced hepatitis. Development of liver injury induced by ConA (10mg/kg, iv) was characterized by measurements of plasma concentration of alanine aminotransaminase (ALT), inflammatory cytokines (IFNγ and TNFα) and by histopathological examination. ConA-induced hepatitis was characterized by an early activation of inflammatory cytokines (IFNγ; from below 0.05ng/ml to 23.72±8.80ng/ml; TNFα; from 0.07±0.01ng/ml to 0.71±0.12ng/ml, 2h after ConA), an elevation of ALT (from 40.65 ±3.2U/l to 5,092.20±1,129.05U/l, 8h after ConA) and by morphological signs of severe liver inflammation and injury (24h after ConA). In mice injected with ConA, NNMT activity in the liver was up-regulated approximately 2-fold to 3-fold, 8–24h after ConAinjection. The concentration of MNA and its metabolites (Met-2PY and Met-4PY) in plasma were elevated approximately 2-fold 8h after ConA injection. Exogenous MNA (100mg/kg, iv) diminished ConA-induced liver injury, and this effect was reversed by an antagonist of the prostacyclin receptor, RO 3244794 (10mg/kg,po). In conclusion, the present study demonstrated that hepatic NNMT activity and MNA concentration in plasma significantly increased during the progression of ConA-induced hepatitis in mice. This response may play a hepatoprotective role compatible with the PGI2-releasing properties of MNA.