The carcinogenic potential of gamma-oryzanol, a drug mainly used for the treatment of hyperlipidaemia, was studied in F344 rats. Groups of 50 males and 50 females were fed a diet containing 0 (control), 200, 600 or 2000 mg gamma-oryzanol/kg body weight/day for 2 yr. Although females in the highest dose group (2000 mg/kg body weight) showed a slight decrease in body weight at 104 wk, there were no treatment-related changes in general condition, food consumption, mortality, organ weight or haematology. Histopathological examination showed various tumours in all groups, including the control group. In the control and 2000-mg/kg groups, high tumour incidences were observed in the testes, pituitary and thyroid of males, and in the pituitary, uterus and mammary gland of females; however, there was no significant increase in the incidence of any tumours between the control and the 2000-mg/kg groups. The findings indicate that under the experimental conditions described gamma-oryzanol was not carcinogenic in F344 rats.