Measurement of chemically induced DNA repair is a means of assessing the ability of a chemical to reach and alter the DNA. DNA repair is an enzymatic process that involves the recognition and excision of DNA-chemical adduct followed by DNA strand polymerization and ligation to restore the original primary structure of the DNA (7). This process can be quantitated by measuring the amount of labeled thymidine incorporated into the nuclear DNA of cells that are not in S-phase and is often called unscheduled DNA synthesis (UDS) (8). Numerous assays have been developed for the measurement of chemically induced DNA repair in various cell lines and primary cell cultures from both rodent and human origin (9). The primary rat hepatocyte DNA repair assay developed by Williams (10) has proven to be particularly valuable in assessing the genotoxic activity and potential carcinogenicity of chemicals (11), (12). Genotoxic activity is often produced by reactive metabolites of a chemical. The in vitro rat hepatocyte assay provides a system in which a metabolically competent cell is itself the target cell for measured genotoxicity. Most other short-term tests for genotoxicity employ a rat liver homogenate (S-9) for metabolic activation, which differs markedly in many important ways from the patterns of activation and detoxification that actually occur in hepatocytes. An extensive literature is available on the use of in vitro hepatocyte DNA repair assays (2, 3, 6, 13-28).