To examine the role of tumor necrosis factor-α (TNFα) in mediating leptin secretion during an immunological challenge, we studied the effects of lipopolysaccharide (LPS) and TNFα on leptin secretion in endotoxin-sensitive C3H/HeOuJ (OuJ) mice, endotoxin-insensitive C3H/HeJ (HeJ) mice, and primary adipocytes cultured from both. Intraperitoneal injection of LPS increased plasma concentrations of TNFα and leptin in OuJ mice, but not in HeJ mice, suggesting a causal relationship between the induction of TNFα and leptin. Consistent with this idea, ip injection of recombinant murine TNFα increased plasma leptin in both OuJ and HeJ mice. To determine whether TNFα induces leptin secretion by acting directly on fat cells, primary adipocytes from OuJ and HeJ mice were cultured in the presence of TNFα or LPS. Whereas LPS was without effect on leptin secretion by adipocytes, TNFα induced a marked increase in the cell supernatant leptin concentration. These data demonstrate that TNFα plays a role in regulating the increase in leptin caused by LPS. Moreover, they show that TNFα can act directly on adipocytes to stimulate leptin secretion. Our results are consistent with the emerging view that leptin is a key hormone coupling immune system activity to energy balance.