Because melatonin is a cerebral vasoconstrictor agent, we tested whether it could shift the lower limit of cerebral blood flow autoregulation to a lower pressure level, by improving the cerebrovascular dilatory reserve, and thus widen the security margin. Cerebral blood flow and cerebrovascular resistance were measured by hydrogen clearance in the frontal cortex of adult male Wistar rats. The cerebrovasodilatory reserve was evaluated from the increase in the cerebral blood flow under hypercapnia. The lower limit of cerebral blood flow autoregulation was evaluated from the fall in cerebral blood flow following hypotensive hemorrhage. Rats received melatonin infusions of 60, 600, or 60,000 ng · kg¹ · h¹, a vehicle infusion, or no infusion (n = 9 rats per group). Melatonin induced concentration-dependent cerebral vasoconstriction (up to 25% of the value for cerebrovascular resistance of the vehicle group). The increase in vasoconstrictor tone was accompanied by an improvement in the vasodilatory response to hypercapnia (+50 to +100% vs. vehicle) and by a shift in the lower limit of cerebral blood flow autoregulation to a lower mean arterial blood pressure level (from 90 to 50 mmHg). Because melatonin had no effect on baseline mean arterial blood pressure, the decrease in the lower limit of cerebral blood flow autoregulation led to an improvement in the cerebrovascular security margin (from 17% in vehicle to 30, 55, and 55% in the low-, medium-, and high-dose melatonin groups, respectively). This improvement in the security margin suggests that melatonin could play an important role in the regulation of cerebral blood flow and may diminish the risk of hypoperfusion-induced cerebral ischemia.