Transcranial Doppler (TCD) is a non-invasive method for measuring blood flow velocity (BFV), and a marker of vessel diameter. In this study, intracranial BFV was investigated, by means of TCD, in patients suffering from probable medication-overuse headache (PMOH). Twenty-three female patients with probable ergotamine- overuse headache (PEOH), 23 female patients with probable analgesic-overuse headache (PAOH), and 15 healthy female controls participated in the study. The mean BFVs of the bilateral middle and anterior cerebral arteries (MCA and ACA) and basilar artery (BA) were measured by TCD. The mean BFVs of the BA and MCA were found to be significantly increased in the PEOH group when compared with those of the PAOH and control groups (p<0.05). No significant differences in BFV of the ACA were observed between any groups (p>0.05). The mean BFV of all the vessels in the PAOH group was found to be lower than that of the control group but no statistical significance was found (p>0.05). Our results show that ergotamine increases BFV via vasoconstriction, especially of the BA and MCA. We also suggest that 5HT1B/1D receptors are mainly localized in the BA and MCA, and that analgesic overuse results in a functional disorder of neuronal receptor and neurovascular reflexes and may cause a reduction of intracerebral vessel tone, leading to vasodilatation.