Plasma leptin and ob gene mRNA levels were increased in mice following bacterial peritonitis, and blocking an endogenous tumor necrosis factor alpha (TNF-alpha) response blunted the increase. However, plasma leptin concentrations did not correlate with the associated anorexia. We conclude that leptin expression is under partial regulatory control of TNF-alpha in peritonitis, but the anorexia is not dependent on increased leptin production.