The effect of high-dose fentanyl on the cerebral vascular response to alterations in mean arterial blood pressure, arterial O2tension (PaO2), and arterial CO2tension (PaCO2) was studied in 28 mongrel dogs using the cerebral venous outflow technique. In 13 animals anesthetized with sodium pentobarbital (30 mg/kg, iv), bolus injection of fentanyl (25 μ/kg, iv) decreased mean arterial blood pressure (MABP) without a change in cerebral blood flow (CBF). In these animals, the response of the cerebral circulation to changes in PaO2, PaCO2, and MABP was determined before and after fentanyl administration. Fentanyl did not alter the increase in CBF caused by hypoxic hypoxia or hypercapnia. The lower and upper limit of cerebral autoregulation determined by hypovolemic hypotension and norepinephrine infusion, respectively, also were unaltered by fentanyl administration. The CBF response to alterations of MABP, PaO2, and PaCO2were studied in another group of 15 dogs anesthetized with fentanyl (100 μ/kg) plus small doses (3–5 mg/kg) of pentobarbital. The CBF response to PaO2, and PaCO2in these animals was not different from that observed in animals anesthetized with barbiturates only. The lower and upper limit of cerebral autoregulation also were not different from that observed in animals anesthetized with barbiturates only. These data suggest that fentanyl in doses sufficient to cause profound analgesia and anesthesia did not alter cerebral responsivity to changes in PaO2, and PaCO2, and MABP.