In 31 experimental groups, 1955 rats were exposed to methyl-, dimethyl-, trimethyl-, and ethylnitrosourea or dimethylphenyltriazene. Altogether, 3102 tumours were registered. 1930 of them were located in the central nervous system. Derived from neurooncological classifications in man, these tumours were subdivided corresponding to a three-step grading scheme. In contrast to brain tumours in man, in rats the degree of dedifferentiation is of minor importance with regard to the clinical course of the cancer disease. In rats CNS tumours lead to death of the animals within a few hours or days after the onset of clinical symptomatology and should be regarded as biologically highly malignant. The grading allows a more exact valuation of the frequency of distinct tumour types in different experimental groups. Following transplacental application, isomorphous neoplasms occur significantly more frequent as compared with anisomorphous ones (grad II and III) which dominate when the carcinogen is given repeatedly to adult animals. Parallels are drawn to human neurooncology.