In the present study, we have investigated the correlation between hepatic fibrosis in rats subjected to bile duct ligation, the numbers of Kupffer and fat-storing cells, and the level of endotoxin in both the portal and systemic circulation. The extent of hepatic fibrosis was measured by morphometry. Kupffer cells were identified by indirect immunoperoxidase staining using ED-2 anti-macrophage antibody. Fat-storing cells were stained with DE-B-5 anti-desmin antibody. Endotoxin levels were determined by the Limulus Lysate test. Following bile duct ligation, connective tissue septa rapidly developed in periportal areas. After 1 week, the volume density of connective tissue had increased from 0.6 +/- 0.1% in control animals to 3.8 +/- 1.1%. After 2 weeks, this volume increased to 19.9 +/- 1.3%, and after 3 weeks to 34.3% +/- 2.7%. The number of periportal fat-storing cells increased 2.8-fold during the first 2 weeks, whereas pericentral fat-storing cells increased only 1.7-fold. After 2 weeks, no further increase was observed. During the first week of bile duct ligation, the number of Kupffer cells increased nearly two-fold. Thereafter, no further increase was detected. In control rats, only two of ten rats showed low amounts of endotoxin in the portal blood. Portal endotoxemia increased with time after bile duct ligation. After 3 weeks, all rats were positive. The measured endotoxin levels were approximately 7 times higher than in control rats. We conclude that the development of fibrosis secondary to experimental bile duct ligation is accompanied by protal endotoxemia, and increases in the numbers of Kupffer and periportal fat-storing cells. We found a significant correlation between portal endotoxemia, the number of Kupffer and fat-storing cells, and the extent of fibrous septa, supporting the view that high endotoxemia levels coincide with Kupffer cell activation and fibrogenesis.