To examine the role of tumor necrosis factor-alpha (TNF alpha) in mediating leptin secretion during an immunological challenge, we studied the effects of lipopolysaccharide (LPS) and TNF alpha 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 TNF alpha and leptin in OuJ mice, but not in HeJ mice, suggesting a causal relationship between the induction of TNF alpha and leptin. Consistent with this idea, i.p. injection of recombinant murine TNF alpha increased plasma leptin in both OuJ and HeJ mice. To determine whether TNF alpha induces leptin secretion by acting directly on fat cells, primary adipocytes from OuJ and HeJ mice were cultured in the presence of TNF alpha or LPS. Whereas LPS was without effect on leptin secretion by adipocytes, TNF alpha induced a marked increase in the cell supernatant leptin concentration. These data demonstrate that TNF alpha plays a role in regulating the increase in leptin caused by LPS. Moreover, they show that TNF alpha 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.