Endothelin (ET) stimulates vasoconstriction and glucose production and mediator synthesis in the liver. Only hepatic endothelial cells express ET-1 mRNA, and during endotoxemia in the intact rat, a ninefold increase in hepatic ET-1 mRNA occurs within 3 h of lipopolysaccharide (LPS) infusion [A. T. Eakes, K. M. Howard, J. E. Miller, and M. S. Olson. Am. J. Physiol. 272 (Gastrointest. Liver Physiol. 35): G605-G611, 1997]. The present study defines the mechanism by which hepatic ET production is enhanced during endotoxin exposure. Culture media conditioned by exposure to endotoxin-treated Kupffer cells stimulated a twofold increase in immunoreactive ET-1 (irET-1) secretion by liver endothelial cells. Transforming growth factor- (TGF-), tumor necrosis factor- (TNF-), LPS, and platelet-activating factor (PAF) were tested for their ability to stimulate cultured liver endothelial cells to secrete irET-1. Although TNF-, LPS, and PAF had no significant effect on ET-1 synthesis, TGF- increased ET-1 mRNA expression and irET-1 secretion. In coculture experiments, treating Kupffer cells with endotoxin caused a doubling of the ET-1 mRNA level in the liver endothelial cells. This increase in ET-1 mRNA was attenuated by a TGF--neutralizing antibody. Hence, a paracrine signaling mechanism operates between Kupffer cells that release TGF- on endotoxin challenge and hepatic endothelial cells in which TGF- stimulates ET-1 mRNA expression and ET-1 secretion; this intercellular signaling relationship is an important component in the hepatic responses to endotoxin exposure.