Two-year toxicology and carcinogenesis studies of benzene were conducted in groups of 50 F344/N rats and 50 B6C3F1 mice of each sex and for each of three exposure doses and vehicle controls. These studies were conducted because of large production volume and widespread human exposure to benzene, because of the epidemiologic association with leukemia, and because previous studies were considered inadequate for determining carcinogenicity. Doses of 0, 50, 100 or 200 mg/kg body weight benzene in corn oil (5 ml/kg) were administered by gavage to male rats, 5 days per week for 103 weeks. Doses of 0, 25, 50, or 100 mg/kg benzene in corn oil were administered by gavage to female rats and to male and female mice for 103 weeks. At week 92 for rats and week 91 for mice, survival was greater than 60% in all groups; most of the animals that died before week 103 had neoplasia. Benzene-associated nonneoplastic or neoplastic effects on the hematopoietic system, Zymbal gland, forestomach, and adrenal gland were found for rats and mice.