We have studied the effect of nitrous oxide on cerebral haemodynamics in 24 healthy male volunteers. Hemispherical cerebral blood flow (CBF) was measured using the xenon-133 inhalation technique, blood flow velocities in the right middle cerebral artery were calculated using transcranial Doppler ultrasound and the pulsatility index (PI)--the inverse of which is theoretically proportional to flow in the vessel under investigation--was derived from analysis of the spectrally analysed velocity pulse wave form obtained from the middle cerebral artery. Each variable was measured with the subject inhaling 100% oxygen (1st baseline), 30% nitrous oxide in oxygen, 100% oxygen (2nd baseline) and 60% nitrous oxide in oxygen. CBF was significantly greater with 30% (0.01 > P > 0.001) and 60% nitrous oxide (P < 0.001) compared with baseline, although the difference between 30% and 60% nitrous oxide was not significant. Changes in 1/PI correlated closely with those in hemispherical CBF. Blood flow velocities increased significantly with 30% (P < 0.001) and 60% nitrous oxide (0.005 > P > 0.001), the difference between 30% and 60% nitrous oxide also being significant (0.005 > P > 0.001). We observed a plateau in the change in CBF caused by nitrous oxide and suggest that this may be explained by activation of intact autoregulative mechanisms in healthy human brain.