The content and activity of cytochrome P4502E1 is increased in the diabetic state, primarily due to stabilization of the P4502E1 mRNA. Chemical inducers such as 4-methylpyrazole (4MP) increase P4502E1 content by stabilization of the protein. Experiments were carried out to evaluate the combined effects of 4MP and streptozotocin-induced diabetes on P4502E1 protein, catalytic activity and mRNA levels. Immunoblots showed an elevated content of P4502E1 after treatment with 4MP or streptozotocin, which was further increased when the two treatments were combined. Similarly, catalytic activity with effective substrates for P4502E1 was increased by the two separate treatments, and further increased by combined treatment. In all treatment groups, catalytic activity was strongly inhibited by antibody against P4502E1. The content of P4502E1 and catalytic activity in the 4MP plus streptozotocin group appeared to be additive of the values for the separate treatments. P4502E1 mRNA levels were elevated by the streptozotocin treatment but not by 4MP treatment; combined treatment with both inducers did not elevate P4502E1 mRNA levels beyond the increase produced in the diabetic state. CCl4 decreased cellular viability in hepatocytes from streptozotocin- or 4MP-treated rats, and increased toxicity was found after treatment with both inducers. These results contrast the mechanisms of induction of P4502E1 by streptozotocin and 4MP, and suggest that each individual mechanism is maintained when the two inducers are administered such that effects on P4502E1 protein and catalytic activity, but not mRNA, are additive of values found for each inducer alone. The diabetic state may be associated with increased sensitivity to toxins which are activated by P4502E1, especially if chemical inducers similar to 4MP, e.g., ethanol, isoniazid are also present.