Abnormal cortical excitability influences susceptibility to cortical spreading depression (CSD) in migraine. Because transcranial direct current stimulation (tDCS) is capable of inducing lasting changes of cortical excitability, we investigated the after-effects of tDCS on the propagation velocity of CSD in the rat. Twenty-five anesthetised rats received either anodal, cathodal or sham tDCS. The stimulation was applied for 20 min at a current strength of 200 microA after the recording of three baseline CSD measurements. Starting 5 min after tDCS, a further three CSDs were elicited and CSD velocity recorded at intervals of 20 min. tDCS and CSD recording was performed under anaesthesia with chloralose and urethane. As compared to the baseline velocity of 3.14 mm/min, anodal tDCS induced a significant increase of propagation velocity during the first post-tDCS recording (3.49 mm/min). In contrast to anodal tDCS, neither cathodal tDCS nor sham tDCS, which consisted of an initial ramped DC stimulation lasting only 20 s, showed a significant effect on CSD propagation velocity. As anodal tDCS is known to induce a lasting increase of cortical excitability in the clinical setting, our results support the notion that CSD propagation velocity reflects cortical excitability. Since cortical excitability and susceptibility to CSD is elevated in migraine patients, anodal tDCS - by increasing cortical excitability - might increase the probability of migraine attack in these patients, even beyond the end of its application.