Autoregulation was tested by regional blood flow (rCBF) and cardiohemodynamic measurements before and after induced systemic arterial hypertension in 16 patients with varying neurological disorders. Hypertension was induced by increasing the arterial blood pressure by an intravenous infusion of Aramine. Seven (Group 1) of the patients had a mean increase in mean arterial pressure (MAP) of 32 mm Hg and had preserved autoregulation while nine (Group 2) with a 56 mm Hg increase in MAP showed complete or mixed loss of autoregulation. Group 1 had a higher baseline mean CBF than did the group with loss of autoregulation. The group with loss of autoregulation also generally had more severe involvement on the cerebral angiogram than did the other. Baseline cardiac index and cardiac work were lower in the group with loss of autoregulation. During Aramine infusion the MAP was increased by 38% in Group 1 and 59% in Group 2. The mean CBF was essentially unchanged in Group 1 but increased 24% in Group 2. When autoregulation is lost, rCBF may increase homogeneously or heterogeneously with some areas increasing while others remained unchanged or even decreased. In four instances there was an intracerebral steal during induced hypertension with a fall in rCBF. Whether or not autoregulation is preserved could be related to: (1) the greater induced increase in MAP in Group 2 than Group 1, (2) greater angiographical involvement with a lower baseline in CBF in Group 2 than in Group 1 or (3) a direct or indirect influence of various cardiovascular factors.