BACKGROUND: Tumor necrosis factor a (TNFa), a cytokine produced at in¯ammatory sites and in adipose tissue, is known primarily for its detrimental effects on insulin action. There is evidence to suggest that TNFa may also in¯uence b-cell function. Leptin is another adipose tissue-derived hormone that might also act on b-cells. OBJECTIVE: We explored the independent and combined effects of TNFa and leptin upon basal and glucose-stimulated insulin transcription and secretion in the HIT-T15 pancreatic b cell line. METHODS: Cells were cultured for 40 h in the presence of near-normal basal (7 mM) or high (16.7 mM) glucose and treated with either TNFa (1, 10 and 50 ngaml) or leptin (10, 50 and 100 ngaml) or both together. Insulin concentrations were measured by radioimmunoassay. Insulin mRNA levels were evaluated by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method, after normalization with b-actin mRNA. RESULTS: TNFa signi®cantly suppressed basal and glucose-stimulated insulin secretion and proinsulin mRNA transcription in a dose-dependent manner, an effect that was more powerful in the presence of high glucose. Leptin also inhibited dosedependent insulin mRNA and protein at both glucose concentrations, but did not appear to further potentiate the suppressive effects of TNFa. CONCLUSION: TNFa suppresses both basal and glucose-stimulated insulin transcription and secretion in HIT-T15 cells, an effect that is enhanced signi®cantly by high glucose. Leptin also independently inhibits basal and glucose-stimulated insulin secretion and transcription but does not modify TNFa effects. These effects might contribute to the abnormalities of glucose metabolism that characterize conditions of increased TNFa andaor leptin production.