Toxicokinetics of 2-mercaptobenzimidazole (MBI) and 2-mercaptomethylbenzimidazole (MMBI), rubber antioxidants with thioureylene structure, were compared after single oral administration in rats. Male Wistar rats received single oral administration of 2, 10, 50 and 250 mg/kg of MBI or MMBI. The serum and urine concentrations of MBI and MMBI were determined by HPLC. MBI and MMBI showed similar Cmax values, but the former disappeared slower in the serum than the latter and resulted in its larger AUC values. Analyses of MBI, MMBI and their desulfurated metabolites in urine suggested that these differences were due to their metabolic elimination rates. On the other hand, MBI and MMBI caused similar acute toxicities, such as loss of locomotive activity, ataxic gait, adoption of prone or side position and coma, being severer with higher serum concentrations at the moment. Similar acute toxicities between MBI and MMBI were explained by similar Cmax values at the same dose. It was suggested from these results that the slower disappearance and larger AUC values of MBI in the serum compared to MMBI might explain the strong thyroid toxicity which has been observed by repeated administration of MBI, but very weak thyroid toxicity by MMBI.