['Transcranial Doppler sonography measures blood flow velocity in basal cerebral vessels with high accuracy and time resolution. Dynamic blood flow tests were used to investigate functional integrity of the cerebral autoregulation (CA) mechanism. It is known from cortical activation studies that velocity indices differ in their appropriateness to measure changes in blood flow velocity. We aimed to compare the peak systolic, end diastolic and time-averaged mean flow velocity indices for their use in measuring the effectiveness of the CA mechanism.', 'We performed the leg cuff test to induce in 15 healthy volunteers (aged 24.8 +/- 0.4 years, 9 males) CA due to a sudden arterial blood pressure decrease. Data from the middle and posterior cerebral arteries were measured continuously with transcranial Doppler ultrasound, and the arterial blood pressure with a noninvasive photoplethysmographic method. After transforming all variables to relative changes, velocity-pressure diagrams were calculated for each velocity-pressure index. Additionally, we calculated the autoregulation index.', 'The step decrease in arterial blood pressure resulted in an initial drop, which is followed by a rapid recovery of cerebral blood flow velocity. The autoregulation index was 5.5 +/- 1. Efficacy of CA is illustrated more accurately by peak systolic velocity-pressure curves, which lie continuously above a passive velocity-pressure relationship assuming CA to be absent.'] Does peak systolic velocity Doppler index reflect most appropriately the dynamic time course of intact cerebral autoregulation?