The dynamic relationships among mean flow velocity, its pulsatile amplitude (FVa), cortical cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were studied in normal rabbits and rabbits with subarachnoid hemorrhage using 8-MHz pulsed transcranial Doppler ultrasound and hydrogen clearance under conditions of systemic hypotension and intracranial hypertension. A two-slope relationship was observed between FVa and CPP with a break point that correlated closely with the lower limit of CBF autoregulation in each animal. Below this CPP break point, FVa varied directly with CPP, and above the break point FVa varied inversely with CPP. In this experimental model, an inverse correlation between FVa and CPP indicates intact CBF autoregulation, whereas loss of that correlation implies exhaustion of autoregulatory reserve. Simultaneous recording and computation of FVa, CPP, and the correlation coefficient between FVa and CPP may be a means of monitoring CBF autoregulation in clinical practice.