Transcranial Doppler sonography (TCD) was used to assess the effects of event rate on cerebral blood flow velocity (CBFV). Fast (30 events/minute) and slow (5 events/minute) event rates were employed in a 40-minute vigilance task that simulated the control of remotely piloted aircraft. As is typical in vigilance tasks, signal detection declined over time but there was no performance difference in conjunction with the two event rate conditions. Nevertheless, CBFV was greater in the fast than in the slow event rate condition and declined significantly with time on task. These outcomes support previous findings of a close tie between CBFV and the vigilance decrement and suggest the possibility that in regard to event rate, CBFV is a more sensitive index of task demand than performance itself