The intravenous injection of LPS rapidly evokes fever. We have hypothesized that its onset is mediated by prostaglandin (PG)E2 quickly released by Kupffer cells (Kc). LPS, however, does not stimulate PGE2 production by Kc as rapidly as it induces fever; but complement (C) activated by LPS could be the exciting agent. To test this hypothesis, we injected LPS (2 or 8 g/kg) or cobra venom factor (CVF, an immediate activator of the C cascade that depletes its substrate, ultimately causing hypocomplementemia; 25 U/animal) into the portal vein of anesthetized guinea pigs and measured the appearance of PGE2, TNF-␣, IL-1␤, and IL-6 in the inferior vena cava (IVC) over the following 60 min. LPS (at both doses) and CVF induced similar rises in PGE2 within the first 5 min after treatment; the rises in PGE2 due to CVF returned to control in 15 min, whereas PGE2 rises due to LPS increased further, then stabilized. LPS given 3 h after CVF to the same animals also elevated PGE2, but after a 30-to 45-min delay. CVF per se did not alter basal PGE2 and cytokine levels and their responses to LPS. These in vivo effects were substantiated by the in vitro responses of primary Kc from guinea pigs to C (0.116 U/ml) and LPS (200 ng/ml). These results indicate that LPS-activated C rather than LPS itself triggers the early release of PGE2 by Kc. liver; fever; portal vein cannulation; cobra venom factor; pyrogenic cytokines FEVER DEVELOPS QUICKLY AFTER the intravenous bolus administration of a pyrogenic dose of LPS to conscious guinea pigs, rats, and other species, but the afferent mechanism that induces this response is controversial. It is generally thought that it is mediated by pyrogenic cytokines produced secondarily in response to the LPS challenge, rather than by its direct action. Tumor necrosis factor-␣ (TNF-␣), IL-1␤, and IL-6 are the major cytokines implicated in this response (9). There is, however, a temporal disconnect between the first appearance of these cytokines and the onset of the febrile response to intravenous LPS; that is, fever appears within 10 -15 min (13, 51) whereas TNF-␣, the first cytokine to appear, is not detectable until 30 min after LPS treatment (18,23,24,39). This temporal discrepancy is less evident after the intraperitoneal administration of low to moderate doses of LPS, when the latency of fever onset is ϳ60 min but becomes evident also when higher doses are administered, when the onset latency approaches that after intravenous LPS (7).We have shown previously that the onsets of the febrile responses to intravenous and intraperitoneal LPS are correlated with the appearance of LPS in the liver's Kupffer cells (Kc) (31), the body's principal clearinghouse of LPS (16,34,45) and source of pyrogenic cytokines (10). As cytokines are not constitutively expressed by Kc and their de novo production occurs after some delay (32), we and others have proposed, to account for the promptness of the febrile response to intravenous LPS, that the peripheral pyrogenic signal could be transmitted to the preoptic-anterior...