Solid tumours are well known to be heterogeneous and contain a significant fraction of hypoxic cells, which are protected against the effects of radiotherapy. A non-invasive method for measuring tissue oxygenation would therefore be useful. The F-19 magnetic resonance signals from perfluorocarbons are sensitive to oxygen concentration. We have used this property to measure tumour oxygenation of the GH3 prolactinoma, RIF-1 fibrosarcoma and SaF sarcoma in mice by fluorine magnetic resonance imaging (MRI) of intravenously injected perfluorocarbons which are taken up by macrophages in the tumour. We have also studied the injection of perfluorocarbons directly into the tumour, which allows less of the tumour to be studied but has a higher success rate and gives values more consistent with Eppendorf polarographic electrode measurements.