The report utilizes knowledge of the regulation of tyrosine aminotransferase (TAT) activity in rat liver as the basis for the development of a model system for investigating the effects of carcinogens on gene expression. A protocol utilizing primary monolayer cultures of adult rat hepatocytes was employed. The addition of dexamethasone resulted in a 5-fold induction of TAT activity; adding glucagon along with dexamethasone gave a 12-fold induction. The chemicals tested for possible effects on TAT induction were aflatoxins B1, B2, G1, G2, 2-acetylaminofluorene, 2-aminofluorene, 7,12-dimethylbenz[a]anthracene, 3-methylcholanthrene, and benzo[a]pyrene. Carcinogens inhibited the induction of TAT activity by dexamethasone alone or with glucagon in a dose dependent manner, and in general there was a correlation between inhibition of TAT induction and in vivo carcinogenic potency. In addition to the inhibition of TAT induction, the carcinogens similarly inhibited RNA synthesis and to a lesser extent, protein synthesis. The inhibition of these biochemical activities did not appear to be due to cell death.