The role of nitric oxide (NO) in cerebral autoregulation is controversial. The purpose of this study was to compare the effects on the lower limit of the cortical cerebral autoregulation of the inhibition of NO synthesis by Nω-nitro-L-arginine (L-NNA) infusion to saline and phenylephrine in pentobarbital-anaesthetized rats. Variations of the cortical cerebral blood flow (CBF), the cortical cerebrovascular resistances, the mean arterial pressure and the lower limit of cerebral autoregulation were compared in three groups: a group pre-treated with L-NNA (n = 8), a group pretreated with saline (n = 8) and a group pretreated with phenylephrine (n = 5). The laser-Doppler flowmetry continuously measured CBF. Controlled haemorrhage was performed after the intravenous infusion of L-NNA, saline, or phenylephrine. The lower limit of cerebral autoregulation of each rat was computed by the least-squares method. The lower limit of cerebral autoregulation was significantly higher after L-NNA infusion (74 ± 5 mm Hg) than after saline (43 ± 3 mm Hg; p < 0.01) or phenylephrine infusions (52 ± 5 mm Hg; p < 0.05). In conclusion, the role of NO on the cerebral autoregulation has been controversial; our results confirm the hypothesis that NO exerts a significant role in maintaining the lower limit of cerebral autoregulation in pentobarbital-anaesthetized r