An increase of more than 50% in cerebral blood flow velocity in the middle cerebral artery was recently reported in hypocapnic volunteers, while inhaling 50% nitrous oxide. We measured cerebral blood flow velocity in the middle cerebral artery in 10 anaesthetized hypocapnic (ETCO2=25 mmHg) patients with brain tumours while administering increasing concentrations of nitrous oxide. At an end-tidal concentration of 50% and 70% nitrous oxide in oxygen, neither mean arterial pressure (base-line: 84±8 mmHg vs. (50% nitrous oxide): 82±9 mmHg and (70% nitrous oxide): 80±8 mmHg) nor cerebral blood flow velocity in the middle cerebral artery (base-line: 32±7 cm s−1 vs. (50% nitrous oxide): 34±8 cm s−1 and (70% nitrous oxide): 34±9 cm s−1) changed significantly. The data from our clinical investigation indicate that administration of increasing concentrations of nitrous oxide to already anaesthetized and hypocapnic patients does not change cerebral blood flow velocity in the middle cerebral artery.