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The relationship between local cerebral blood flow (measured by the hydrogen clearance technique) and stepwise reductions in mean arterial pressure was studied in 8 anesthetized cats. The relationship between pial arteriolar caliber and hypotension was studied in a further 5 cats. Hypotension was induced by graded hemorrhage. Autoregulation maintained a fairly constant cerebral blood flow over the arterial pressure range 60-120 mm Hg. At mean arterial pressures below 60 mm Hg, cerebral blood flow fell with decreasing arterial pressure. Pial arteriolar and arterial caliber increased as mean arterial pressure was decreased. In the smaller arterioles (≥ 50 μ in diameter at a mean arterial pressure of 100 mm Hg), dilatation was maximal (average of 93%) in the arterial pressure range 30-39 mm Hg. The maximal dilatation was less (+56%) in the larger arterioles and small arteries (≥ 50 nm in reference diameter), but occurred in the same arterial pressure range (30-39 mm Hg). Thus, the lower limit of cerebrovascular autoregulation (˜ 65 mm Hg) occurred at a significantly higher pressure than that at which the pial vessels were maximally dilated (˜ 35 mm Hg). Therefore, it would appear that the lower limit of autoregulation should not be equated with maximal pial vasodilatation, as it has tended to be in the past, but with the arterial pressure at which the cerebral dilatation responses can no longer compensate sufficiently for the decreasing perfusion pressure.