A susceptibility contrast agent which does not pass into the extra-cellular space was used to study the effect of changes in the relative cerebral blood volume (CBV) on the haemodynamic response to anoxia, for both normal and ischaemic brain tissue, in a rat model of acute focal ischaemia. In non-ischaemic tissue a strong CBV component was observed in the haemodynamic response, both during and after anoxia. During anoxia the change in the CBV of the non-ischaemic tissue was estimated to be 40% in the caudate putamen and 70% in the frontal-parietal cortex. For severely ischaemic tissue (ischaemic caudate putamen) there was no change in the CBV during anoxia while in areas of moderate ischaemia (ischaemic frontal parietal cortex) a change of 20% was observed. The effect of the contrast agent on spin-echo images was consistent with a small reduction in the microvascular blood volume of the ischaemic tissue.