Abstract
Male and female F344 rats were continuously administered N-methyl-N-nitrosourea (MNU) in their drinking water at concentrations of 200 or 100 ppm, and both sexes of ACI/N rats were given MNU at a concentration of 200 ppm. By the 42nd week of the experiment, high incidences of brain/spinal cord tumors were observed in both strains of rats. Histologically, many of them were astrocytomas or anaplastic astrocytomas. In addition, malignant neurinomas were also detected in the spinal nerve roots and trigeminal nerves, although their incidences were rather low. There was no difference in the type and incidence of these neurogenic tumors between the two strains of rats. Tumors of the tongue and esophagus were mainly observed in the high-dose group of F344 rats and those of the glandular stomach were observed in the low-dose group of F344 rats. In ACI/N rats, tumors of the heart and renal pelvis were detected. The organ-specific carcinogenicity of MNU in these two strains of rats was compared with that of MNU in Donryu rats. It was demonstrated that organ specificity of MNU given orally was influenced not only by the strain of rats but also by the dose level.