Inhibition of glucose-induced insulin secretion by interleukin-1β (IL-1β), or IL-1β plus tumour necrosis factor-α (TNF-α), was less marked when rat islets of Langerhans were cultured for 12 h with these cytokines in L-arginine-free medium as opposed to medium containing L-arginine (1 mM). Inhibition of secretion by IL-1β was further alleviated when islets were maintained in L-arginine-free medium supplemented with N-ω-nitro-L-arginine methyl ester (NAME), while synergism between IL-1β plus TNF-α was completely abolished. Tissue culture medium nitrite levels were raised in islets treated with IL-1β or TNF-α (48 h). Cytokine-stimulated nitrite production was not observed in islets cultured with NAME (1 mM). In conclusion, an L-arginine-dependent nitric oxide generating mechanism is involved in the inhibition of insulin secretion by IL-1β, and accounts for the phenomenon of synergism between IL-1β and TNF-α.