Introduction: Continuous non-invasive monitoring of cerebral blood flow (CBF) may be important during anaesthesia and several options are available. We evaluated the CerOx monitor that employs ultrasound tagged near infrared spectroscopy to estimate changes in a CBF index (CFI).Methods: Seven healthy males (age 21-26 years) hyperventilated and were administered phenylephrine to increase mean arterial pressure by 20-30 mmHg. Frontal lobe tissue oxygenation (S_cO₂) and CFI were obtained using the CerOx and mean blood flow velocity in the middle cerebral artery (MCAv_mean) was determined by transcranial Doppler. Blood flow in the internal and external carotid artery (ICAf and ECAf) was determined using duplex ultrasonography and forehead skin blood flow (SkBF) and oxygenation (S_skinO₂) by laser Doppler and white light spectroscopy.Results: During hyperventilation MCAv_mean and ICAf decreased by 44% (median; interquartile range 40-49; p = 0.016) and 46% (40-53; p = 0.03), respectively. Conversely, CFI increased by 9% (2-31; p = 0.016), while no significant change was observed in S_cO₂. SkBF increased by 19% (9-53; p = 0.016) and S_skinO₂ by 6% (1-7; p = 0.047), although ECAf was unchanged. Administration of phenylephrine was not associated with any changes in MCAv_mean, ICAf, ECAf, S_cO₂, SkBF, S_skinO₂, or CFI.Conclusion: The CerOx was able to detect a stable CBF during administration of phenylephrine. However, during hyperventilation MCAv_mean and ICAf decreased while CFI increased, likely due to an increase in superficial tissue oxygenation. Thus, CFI does not provide an unbiased evaluation of changes in CBF.