Background. We developed an MRI protocol to measure cerebrovascular diameter and blood flow velocity, and if we could detect cerebrovascular alterations after SAH and their impact on cerebral ischaemia. Method. SAH was induced in 15 Wistar rats by means of the endovascular filament method; 6 other rats served as control. MRI measurements were performed on a 4.7T NMR spectrometer 1 and 48 hours after SAH and 9 days thereafter. Diffusion-weighted and T2-weighted images were acquired to detect cerebral ischaemia. The arterial spin labelling method was used to measure CBF. MR angiography was used to measure vessel diameter and blood flow velocity, from which the arterial blood flow was calculated. Findings. The ischemic lesion volume increased between 1 and 48 hours after SAH from 0.039 to 0.26 ml (P = 0.003). CBF decreased from 53.6 to 39.1ml/100g/min. The vessel diameter had narrowed, the blood flow velocity diminished as did the arterial blood flow in most vessels, but only the vasoconstriction in the right proximal ICA reached significance (0.49mm to 0.43mm, P = 0.016). Baseline values were restored at day 9. Conclusions. We showed that it is feasible to detect alterations of in-vivo vessel diameter and blood flow velocities and their consequences for brain damage after experimental SAH in the rat. The growth of the infarct volume between day 0 and 2 after SAH and the parallel vasoconstriction suggest that delayed cerebral ischaemia after SAH occurs in rats and that this may be caused by vasoconstriction.