Glycidol, a simple aliphatic epoxide, was administered by gavage in water to groups of male and female F344/N rats and B6C3F1 mice. Rats received 0, 37.5 or 75 mg kg-1 and mice received 0, 25 or 50 mg kg-1 daily, 5 days per week for 2 years. Exposure to glycidol was associated with dose-related increases in the incidences of neoplasms in numerous tissues in both rats and mice. Survival of rats that received glycidol was markedly reduced compared to the control because of the early induction of neoplastic disease. In male rats, mesothelioma arising in the tunica vaginalis and frequently metastasizing to the peritoneum were considered the major cause of early death. Early deaths in female rats were associated with mammary gland neoplasms. Survival of female mice that received 50 mg kg-1 was lower than the control after week 101 due primarily to euthanasia of moribund animals with mammary gland neoplasms. Survival of male mice and female mice that received 25 mg kg-1 was comparable to the control. In mice, exposure to glycidol was associated with increased incidences of neoplasms of the harderian gland in males and females, the forestomach in males and the mammary gland in females.