We evaluated cytotoxic effects of different unconjugated and glycine- and taurine-conjugated bile salts (BS) against bile duct epithelial cells in isolated bile ductule fragments and isolated perfused rat liver. Ultrastructural morphometric studies were performed in polarized rat bile ductule fragments exposed in vitro to increasing concentrations (10-100 micromol/L) of lithocholate (LCA), deoxycholate (DCA), chenodeoxycholate (CDCA), cholate (CA), ursodeoxycholate (UDCA), their taurine-conjugates, and glycoconjugates of cholic (GCA) or chenodeoxycholic acid (GCDCA) for 20, 30, or 75 minutes. To evaluate the cytotoxicity of unconjugated hydrophobic bile salts against biliary epithelium (BDE) in the whole liver, livers were isolated from rats with impaired taurine-conjugation capacity (beta-alanine treatment) and perfused for 70 minutes with 2 micromol/min LCA (n = 6), CDCA (n = 6), CA (n = 6), or 0.5 micromol/min tauro-LCA (n = 4). In isolated bile ductule fragments, hydrophobic unconjugated bile salts (LCA, CDCA, DCA) induced a marked damage of intracellular organelles, mainly mitochondria. The damage started at a concentration of 10 micromol/L and became prominent at concentrations higher than 50 micromol/L. No damage of the apical and basolateral membrane was seen and tight junctions appeared intact. UDCA, taurine and glycoconjugated bile salts failed to induce any evident ultrastructural alteration. In taurine-depleted isolated livers, perfused with LCA, CDCA, or CA, bile duct epithelial cells showed no evidence of intracellular damage, despite the increased biliary excretion of unconjugated BS. Marked alterations of the apical cell membrane were seen only in livers perfused with LCA and in isolated segments of the biliary epithelium. In contrast with biliary epithelium, hepatocytes showed prominent subcellular damage with CA and CDCA, and profound alterations of the canalicular membrane with LCA and tauro-LCA. We have shown that, in vitro, BDE cells are not damaged by taurine- or glycine-conjugated BS, but they are very sensitive to cytotoxicity of hydrophobic unconjugated BS. Such sensitivity is not present in the whole liver, probably because of the specificity of BS transport processes, the microvascular architecture of the bile ductal system, and the presence in bile of a physiological surfactant, such as phospholipids.