Based on past biochemical and physiological data s'uggesting that the Golgi apparatus is in some way involved in transcellular transport of hydrophobic bile acids and in biliary lipid excretion, we tested this concept by examining Golgi complex morphology after exposure to a high, prolonged flux of taurocholate (TC) and taurodehydrocholic acid (TDHC). Standard bile flstula rats were infused intravenously with TC and TDHC after reducing the pool of natural bile acids by draining bile via the - fistula for 3 hours. In one group the bile was drained overnight and infused with saline. Sham operated animals were used as controls. 3-d reconstruction: Electron micrographs at a final magnification of 15,000X were recorded from, 16 sequential sections from each condition. The hepatocyte plasma membrane, nucleus, Golgi membranes and lipid-bearing vesicles were digitized into an IBM PC, using programs developed by Drs. Young and Hessler at the NCMIR. Volumes were quantified using ANALYZE (Robb and Barillot,1989). This work was all performed at the NCMIR. Statistical Analysis: ANOVA and Neuman-Keuls tests were used. Sixteen sequential sections of one hepatocyte from each conditions were measured. The Golgi stacks for the TC-infused rats represented 0.58% of the cytoplasmic volume; by contrast, in TDHC-treated animals, the Golgi stacks occupied 1.91% of the total area; overnight-drained animals rendered 1.43%; and 1.25% was obtained for the Sham rats. When vesicles were measured, the TDHC-infused group showed a 6 fold increase (1.08%) compared to the overnight drained rats (0.17%) and occupied an area 3X greater than in Sham rats (0.36%). To confirm these results, measurements were performed on micrographs taken from 10 periportal hepatocytes using one section; three animals per condition were studied with a total of 30 hepatocytes per group. The cross sectional area of the stacks, expressed as percent of the total cell area, was 0.68 % for TC; 0.80% for Sham; 1.28 % for TDHC and 0.90 % for the overnight control. TC was significantly less, p<0.05, than TDHC. When Golgi-associated vesicles were analyzed, TDHC was significantly greater, p<0.05, than the other conditions. Results: The most noteworthy ultrastructural findings in the livers exposed to a high flux of biliary acids occurred in TDHC-treated animals. In this group, both stacks and vesicles were more numerous and both organelles were dispersed throughout the cytoplasm. In the TC treated group, fragmentation and dilation of the Golgi stacks, with diminished Golgi-related vesicles, was prominent. In contrast, the Golgi from livers in which the bile acid pool was depleted by overnight drainage, showed parallel arrangement of elongated Golgi stacks - This study shows that a high flux of taurodehydrocholic acid through the hepatocyte causes structural changes of the Golgi apparatus with a considerable increase in the surface area occupied by both Golgi membranes and vesicles, whereas in taurocholate, disorganization and decrease in the Golgi aggregate size was the hallmark. These findings add support to the concept that the Golgi apparatus is involved in bile acid stimulated biliary lipid secretion.