The potential toxicity and carcinogenicity of thiabendazole (TBZ) was investigated in F344 rats. TBZ was given in pelleted diets at different dietary levels (0, 0.05, 0.1, 0.2 or 0.4%) to groups of 30 rats of each sex for 104 wk. Throughout the study, a marked decrease in body-weight gain was observed in males and females fed 0.4% TBZ. Transitional cell hyperplasia of the renal pelvis and/or papilla was observed in dosed groups of each sex with significant positive trends. Low incidence of transitional cell papilloma or carcinoma of the urinary system was recorded in the dosed groups. Preputial gland adenoma occurred in males treated with 0.4% TBZ, and the incidence was significantly higher than in controls. There was no clear-cut evidence of the carcinogenic activity of TBZ in the urinary system of male and female rats.