Immunologic mechanisms contribute to the development of pyrogenic tolerance to bacterial endotoxins. The present studies test the concept that the hepatic Kupffer cell is the “target” tissue protected by these immune mechanisms. The pyrogenic response of healthy acclimatized New Zealand rabbits to a purified Escherichia coli endotoxin administered by ear vein was characterized by a classic biphasic febrile response, an initial early phase (60- to 90-min peak) and a later second phase (180-min peak). Tolerance induced by repetitive daily ear vein administration of the endotoxin resulted in ablation of the second febrile phase, but only minimal reductions of the early phase. These responses were then contrasted with those induced when the E. coli endotoxin was presented directly to the hepatic Kupffer cell by perfusion through chronically implanted hepatic portal venous cannulae. Direct hepatic perfusion with endotoxin resulted in diminution or ablation of the early phase of fever in both non-tolerant and tolerant animals. This effect was seen in 3 of 12 non-tolerant and all of 21 tolerant animals. Reticuloendothelial blockade with thorotrast consistently restored the early fever phase following portal vein administration of the endotoxin in the tolerant animals. In contrast to the early febrile response, direct hepatic perfusion with endotoxin enhanced the second febrile phase in the non-tolerant animals; the second febrile phase, however, remained markedly inhibited in 17 of the 21 tolerant animals tested. Analysis of these findings indicates: 1) extra-hepatic mechanisms dominate the first phase of the biphasic febrile response to bacterial endotoxin in the rabbit, 2) hepatic mechanisms dominate the second phase of the biphasic febrile response to endotoxin, 3) the liver of the tolerant rabbit clears directly perfused endotoxin more efficiently than the liver of the non-tolerant animal and 4) despite the more efficient uptake of directly perfused endotoxin by the liver of the tolerant rabbit, the second phase of fever usually remains markedly inhibited. The findings fully support the concept that tolerance to the pyrogenic activity of bacterial endotoxin is based upon increased uptake of the toxin by hepatic Kupffer cells which have become refractory to further release of endogenous pyrogen.