The effects of X-radiation on the Nembutal-induced redistribution of the cardiac output in two types of transplanted mouse tumours and some normal mouse tissues have been investigated, using rubidium-86 and 125I-human serum albumin. Irradiation causes an increase in 86Rb uptake (relative blood perfusion) by the tumours of anaesthetized mice, but has little or no effect in non-anaesthetized mice. The increase is dose- and time-dependent. Tumour plasma space is not significantly affected by radiation and Nembutal. Muscle blood perfusion is severely decreased in anaesthetized mice and is not affected by radiation, at least within the time limits of the experiments. This means that radiation-induced functional vascular changes in normal and neoplastic tissues follow different time courses. On the basis of the present results, and of the results of other authors, it is argued that irradiation damages the vasculature of tumours in such a way that it becomes more sensitive to changes in systemic blood pressure
