Gliomas were induced transplancentally by the administration of ethylnitrosourea to pregnant rats on the 15th day of gestation. The fine structure of neurones involved in these tumours was studied in order to assess the changes caused by neoplasia. The severity of the neuronal damage depended on their location, Those within the tumours being more affected than those in the neighbouring brain. The histological type and grade of malignancy also influenced the changes: periventricular pleomorphic gliomas and ependymomas of high-grade malignancy caused the most severe alterations. Neurones displayed a spectrum of appearances from apparent normality to complete necrosis. Both nucleus and cytoplasm were affected: of the cytoplasmic organelles the rough-surfaced endoplasmic reticulum and mitochondria showed the most striking changes. There were many lipofuscin granules and autophagic vacuoles. Axons underwent advanced degeneration: swollen mitochondria, disrupted vesicles and vacuoles, disintegrated microtubules, irregular filaments and unidentified debris were sometimes seen. Disintegration of myelin sheaths frequently occurred with consequent engulfing of their debris by macrophages and reacting astrocytes. The neuronal satellites--mainly oligodendrocytes--were often replaced by neoplastic glial cells. Hydrolytic 'marker' enzymes were demonstrated in neurones at various stages of degeneration. In all cases neurones displayed low acid phosphatase activity, while thiamine pyrophosphatase activity decreased with increasing neuronal degeneration.