Rapid sampling microdialysis (rsMD) directed towards the cerebral cortex has allowed identification of a combined time-series signature for glucose and lactate that characterizes peri-infarct depolarization in experimental focal ischaemia, but no comparable data exist for 'classical' cortical spreading depression (CSD) associated with hyperaemia in the normally perfused brain. Here, we examined the rsMD responses of dialysate glucose and lactate to five hyperaemic spreading depressions induced with intracortical microinjections, typically of 1 mol/L KCl, in open-skull preparations in five cats under chloralose anaesthesia. Depolarization was verified with microelectrodes, and laser speckle flowmetry was used to examine propagation of the events and perfusion responses near the MD probe. Ten minutes after depolarization, dialysate glucose fell and lactate rose by 28% and 58% respectively. There was no recovery of dialysate glucose 30 mins after depolarization. Mean baseline indicative cerebral blood flow was 25.5+/-4.1 mL/100 g/min and mean maximum hyperaemic increase was by 29.6+/-6 mL/100 g/min; hyperaemia remained present 30 mins after CSD. As CSD events are repetitive, frequent, and often clustered temporally in human acute brain injury, these results indicate a high risk of depletion of extracellular glucose in association with depolarization events of a pattern previously thought to be largely benign.