In the present study, we investigated the effect of remifentanil on cerebral blood flow velocity (CBFV). We investigated 20 patients (ASA physical status III) scheduled for elective coronary artery bypass graft surgery. Anesthesia was induced with remifentanil 5 microg/kg IV (Group 1, n = 10) or 2 microg/kg IV (Group 2, n = 10) and was maintained with 3 microg x kg(-1) x min(-1) IV (Group 1) or 1 microg x kg(-1) x min (-1) IV (Group 2). Pancuronium (0.1 mg/kg IV) was administered for muscle relaxation. Assisted ventilation followed by controlled ventilation via a mask was performed with the PaCO2 kept constant. Mean cerebral blood flow velocity (Vmean) was measured in the middle cerebral artery using a transcranial Doppler sonography system. Mean arterial pressure (MAP) was kept constant by the IV administration of norepinephrine. Measurements were made at baseline and every minute after remifentanil infusion for 10 min. Data were analyzed by using analysis of variance and a post hoc t-test (P < 0.05). Heart rate, MAP, and PaCO2 did not change over time in either group. Vmean did not change in Group 2. In contrast, there was a 31% decrease of Vmean in Group 1 (P < 0.05). The results show that large-dose, but not moderate-dose, remifentanil reduces CBFV unrelated to any changes in systemic hemodynamics in isocapnic cardiac patients. CONCLUSIONS Transcranial Doppler sonography was used to monitor remifentanil-induced changes in cerebral perfusion. We found that large doses of remifentanil reduced cerebral blood flow velocity despite constant perfusion pressure. This may implicate a central mechanism for cerebral hemodynamic effects of remifentanil.