2-Mercaptobenzimidazole (MBI), a rubber antioxidant, is known to exhibit potent thyroid toxicity in rats, whereas its methylated derivatives are much less toxic. To characterize this methyl-substituent effect on the thyroid toxicity of MBI, comparative toxicokinetic analyses have been conducted in the present study. MBI and the MMBIs [4-methylated MBI (4-MMBI) and 5-methylated MBI (5-MMBI), and a 1:1 mixture of these 4- and 5-methylated isomers (MMBI mix)] suspended in corn oil were repeatedly administered (at 0.3-0.6 mmol/kg) to male Wistar rats by gavage once daily for 2 weeks. After the first and last administrations, blood and urine samples were collected, and the levels of unchanged compounds and their desulfurated metabolites were determined by high performance liquid chromatography. After repeated oral administration (roa), the C(max) and area under concentration-time curve (AUC) of MBI were markedly increased, while the MMBIs essentially were cleared from the blood within 10 h. After roa, the C(max) and AUC of 4-MMBI decreased markedly, suggesting metabolic enzyme induction. However, the toxicokinetic parameters of 5-MMBI were not markedly altered by roa. The inhibitory potencies (IC(50)) against lactoperoxidase of MBI, 4-MMBI, and 5-MMBI were 20.6 micro M, 45.6 micro M and 31.6 micro M, respectively. Thus, we suggest that the marked decrease of thyroid toxicity by methyl substitution of MBI is caused mainly by a decrease in systemic exposure to the compounds and partly by a decrease in inhibition of thyroid hormone synthesis.