An L-arginine-dependent pathway, by which L-arginine is metabolised to citrulline and nitrogen oxides, has been recently identified in some cell types. In cultured rat lung fibroblasts the presence of L-arginine was necessary for the production of nitrite to be induced by rat recombinant interferon-gamma and synergistically enhanced by lipopolysaccharide and interleukin-1 beta. Lipopolysaccharide and interleukin-1 beta did not induce nitrite biosynthesis by themselves. Biosynthesis was apparently dependent on tetrahydrobiopterin, since it could be blocked by diaminohydroxypyrimidine, an inhibitor of tetrahydrobiopterin synthesis. Dexamethasone blocked nitrite production by a receptor-mediated mechanism. These data indicate that rat lung fibroblasts express an L-arginine-dependent nitric oxide synthase which can be induced by some mediators of inflammation.