Background: Intraoperative intracranial pressure (ICP) control continues to be a challenge for anaesthetists during craniotomies. Although many standard brain-dehydrating protocols are available, they may be ineffective in certain surgical situations and may result in harm either to the systemic or cerebral circulation. Sphenopalatine ganglion block (SPGB) can reverse the vasodilatory effects of anaesthesia during craniotomy. Methods: This prospective randomised study was carried from June 2020 to February 2021. Fifty-two patients were randomly allocated into two groups, the block group (B) and the non-block control group (Non). Twenty-six patients were enrolled in the (B) group and received a bilateral transnasal SPG block with 2% lidocaine using a hallow culture swab prior to anaesthesia induction. Intraoperative monitoring was performed using standard American Society of Anesthesiologists (ASA) monitors in addition to invasive monitoring using intra-arterial cannulas and jugular venous bulb catheters. Subdural ICP monitors were also employed. The arterio-jugular oxygen difference in mmol/l (AjvDO2) was then calculated. Mean flow velocity cm/s (MFV) and pulsatility index (PI) were monitored in both groups using Transcranial Doppler. Haemodynamic data were recorded every 30 min from induction of anaesthesia until the closure of the dura. Results: There was a significant difference in ICP prior to the dural opening between the block group (B), mean sd 7.58 1.47, and the non-block group (Non), mean sd (11.69 1.72), p-value < 0.001. There was no significant difference in MFV between (B) group, mean sd 72.65 2.28 and (Non) group, mean sd 71.19 3.09 before intubation (baseline values). While there was a significant difference after intubation between block group, mean sd 72.12 1.77 and non-block group, mean sd 74.62 5.07, pvalue = 0.02. There was an insignificant difference between (B) and (Non) groups before intubation regarding PI values, while PI was significantly higher in (B) group than the (Non) group after intubation where mean sd was 1.17 0.05 versus 0.96 0.09, respectively, p-value = 0.001. There was no significant difference regarding cerebral oxygenation between the groups. Conclusions: SPGB can control factors that increase CBF during anaesthesia by the block of parasympathetic vasodilatory fibres to the arterial system in the anterior cerebral circulation, while neither hindering cerebral venous drainage nor impairing cerebral oxygenation, as it gives no supply to cerebral veins and does not affect basal CBF. Additionally, it does not affect systemic circulation. C 2021 Socie´te´ franc¸aise d’anesthe´ sie et de re´animation (Sfar). Published by Elsevier Masson SAS. All rights reserved.