Hyperventilation results in a fall in carbon dioxide concentration, a fall in cerebral blood flow, and slowing of activity on the electroencephalogram. The temporal relationship and duration of these responses are uncertain, and were investigated using simultaneous monitoring of cerebral blood flow velocity and of the electroencephalograph, with end-tidal carbon dioxide monitoring. Sixteen patients and 9 normal volunteers were studied. Cerebral blood flow velocity in the middle cerebral artery was measured using transcranial Doppler sonography during 3 minutes of hyperventilation and during a 3-minute recovery period. Electroencephalographic recordings were rated by both visual score and measurement of the dominant posterior frequency. End-tidal expired carbon dioxide tension was monitored during the same hyperventilation protocol in the volunteers. Flow velocity fell rapidly during active hyperventilation. Electroencephalographic slowing closely correlated with the decrease in flow velocity (r = 0.86), but lagged behind it. In healthy volunteers capnographic records showed a very tight coupling between end-tidal carbon dioxide concentration and flow velocity (r = 0.94). Three minutes after hyperventilation, carbon dioxide concentration, cerebral blood flow velocity, and electroencephalographic activity were still not back to the resting state. The fall in both cerebral blood flow velocity and carbon dioxide concentration are related to but precede electroencephalographic slowing. The abnormalities persist for at least 3 minutes after hyperventilation and this must be taken into account in clinical electroencephalography. Transcranial Doppler sonography is well suited to monitoring short-term changes in the cerebral circulation.