Tumor necrosis factor-alpha (TNF) is an extremely potent cytokine which is involved in the pathogenesis of a number of diseases. Interruption of its synthesis can result in a reduction of inflammation and subsequent pathology. A new experimental drug pirfenidone (5-methyl-L-phenyl-2-(1H)-pyridone, trade name: Deskar) has been reported to have beneficial effects for the treatment of certain fibrotic diseases. The present study describes the inhibition of TNF in vitro as well as the inhibition of circulating TNF in vivo by pirfenidone. Isolated, thioglycollate-induced peritoneal macrophages (Mphi) from C57BL/6 mice were exposed to either lipopolysaccharide (LPS) or mannosylated bovine serum albumin then incubated with 0.1-0.9 mg/ml of pirfenidone. This substance inhibited the production of TNF in a dose-dependent manner as measured by ELISA. One i.p. injection of either 100 or 200 mg/kg pirfenidone inhibited the induction of circulating TNF following a single i.v. injection of LPS. Endotoxin shock was induced in mice using an i.p. injection of galactosamine and LPS. The higher dose of pirfenidone (200 mg/kg) completely inhibited shock and subsequent mortality. Lower doses of pirfenidone or administration either prior to or post challenge only partially inhibited symptoms. These results indicate that pirfenidone is able to inhibit both TNF induction and subsequent endotoxin shock. Additional studies are warranted to establish this drug as a potential treatment for diseases where TNF plays a major role.