Preliminary analysis of clinical chemistry results from 25 subchronic studies performed for the NTP has revealed that measurement of total bile acid concentration in the serum of rats is a sensitive and specific indicator of decreased hepatobiliary function. Although increased concentrations of bile acids frequently correlate with increased activities of hepatocellular enzymes in serum, in many studies, changes in concentrations of bile acids are often the only indication of treatment- induced hepatobiliary disease. For several years, we have been exploring the theory that alterations in concentrations of individual bile acids in laboratory animals can be used to identify specific lesions in the enterohepatic circulation and may initiate certain types of hepatic damage. A HPLC/enzymatic assay that permits the identification and quantitation of picomolar concentrations of individual bile acids in biologic samples has been developed in-house. In initial investigations in male and female rats in which 12 separate treatments were used (selection was based on the ability of each to disrupt a selected component of the enterohepatic circulation), analysis of the concentrations of individual bile acids permitted the correct identification of the treatment an animal received with approximately 95% accuracy. In 1990, studies were completed which demonstrated the promotional activity of a primary bile acid, chenodeoxycholic acid, on hepatocellular foci in the rat and which identified specific, changes in the bile acid profile of rata treated with a peroxisome proliferator. The identification of an unknown bile acid, which becomes the predominant from in rats with a specific type of liver disease, is progressing through use of liver slice studies, enzyme analyses, and NMR and mass spectroscopy. In other efforts, bile acid studies are being conducted using rat liver slices. Initial studies using this technique have resulted in the selection of methods, defined media, and incubation times. A series of in vitro experiments has been initiated to explore the effects of bile acids and hepatotoxic chemicals on hepatocellular and microsomal enzymes and on the metabolism of bile acids. Using a HPLC assay with a post-column enzymatic reaction, the bile acid profile in approximately 200 samples has been determined this fiscal year.