Gradient echo magnetic resonance (MR) imaging was used to demonstrate propagating waves of cortical spreading depression (SD) in the anaesthetised rat. SD was initiated by remote perfusion with 150 mM KCl applied for 0.5-2 min to the left parietal cortex. Gradient echo MR images were obtained every 12-30 s in either a vertical coronal section or a horizontal section including the superficial cortex in plan view. Within 2 min of application of KCl, we observed a zone of increased signal intensity (3-15%) on the MR image, up to 2 mm across, lasting approximately 1 min and propagating away from the site of initiation. The mean velocity for 27 of such waves seen in seven animals was calculated to be 2.79 mm/min, with means (+/- SD) in individual animals averaging 2.90 +/- 0.46 mm/min (n = 7). Increased signal intensity in gradient echo images has been attributed to an increased level of oxygenation within the venous blood. Our results are consistent with this interpretation although other physiological changes during SD may also contribute to the signal changes.