In a survey of 75,000 brains from 23 inbred mouse strains studied during research work on scrapie, astrocytoma was by far the commonest neuroectodermal tumour. It occurred in VM and BRVR mice at a 1% incidence, but was virtually absent from other strains. In VM mice it occurred nearly twice as often in males as in females, sometimes causing neurological signs of circling, depression or spinal paralysis, but more often showing no clinical effects and being discovered only during routine neuropathology carried out for other purposes. Astrocytomas occurred in mice in the spinal cord or in the brain, with equal frequency in the fore-, mid- and hindbrain. A dual origin has never been confirmed, but astrocytomas occasionally co-existed with other central nervous system tumours. Macroscopically they cause no distortion of the brain; this contrasts with the rare oligodendroglioma, in cases of which the brain is sometimes swollen and misshapen. Astrocytomas characteristically spread through white-matter tracts, but in some grey-matter localities neoplastic astrocytes can co-exist with viable neurons, suggesting that they may subserve some functions of normal astrocytes. The astrocytomas found in this study often co-existed with scrapie lesions. Scrapie-induced vacuolation penetrated some astrocytomas in grey matter, and the specific scrapie-associated argyrophilic (amyloid) plaques occurred in the middle of an astrocytoma in several mice. However, there is no relationship between the occurrence of astrocytoma and scrapie infection, and the neoplasm also occurs independently both of the Sinc gene, which controls the pathogenesis of scrapie, and of the histocompatibility type of the strains concerned. The spontaneous VM astrocytoma is transplantable intracerebrally and, in the early series of in vivo subpassages, it only grew in the brains of mice of the strain of origin and its crosses. The age occurrence of astrocytoma and the incidence of other CNS tumours in mice are discussed in this paper.