The process of vascularisation was studied in transplanted astrocytomas in BD-IX rats. The development of blood vessels was followed from the earliest signs of angiogenesis throughout tumour growth. On the basis of tumour vasculature, 3 consecutive stages of tumour growth could be distinguished; avascular, early vascular and late vascular. The tumours grew to a diameter of about 1 mm during the avascular stage after which new capillary sprouts began to penetrate the tumours. This resulted in an homogeneous vasculature of small immature capillaries up to about 5 micrometers in diameter characteristic of the early vascular growth stage. During this stage the tumours reached a diameter of about 4 mm and their vasculature consisted of capillaries similar to those seen in embryological cerebral vascularisation. During the subsequent late vascular stage of growth, continued endothelial proliferation led to an increase in blood vessel diameter up to 50 micrometers in some cases. The vessels varied in shape and size; this vascular pleomorphism and the abnormal morphological features associated with glioma vasculature were typical of the late vascular stage.