Cerebral blood flow is maintained constant over a range of cerebral perfusion pressures by cerebral autoregulation. Impaired cerebral autoregulation may be important in the pathogenesis of cerebral ischaemia. The mechanisms mediating normal cerebral autoregulation in humans are poorly understood. We used a recently described transcranial Doppler technique, which allows non-invasive measurement of dynamic cerebral autoregulation, to test the hypothesis that nitric oxide mediates cerebral autoregulation. The rate of rise of middle cerebral artery blood flow velocity, compared with that of arterial blood pressure, was determined following a stepwise fall in arterial blood pressure, in order to calculate an autoregulatory index. The effect of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) on dynamic autoregulation was compared with that of noradrenaline titrated to result in a similar rise in blood pressure. Six healthy subjects were studied in each group. The mean (S.D.) change in autoregulatory index following noradrenaline at a similar pressor dose was significantly greater than the change following the L-NMMA bolus: 1. 1 (1.2) compared with -0.8 (0.8) for the left middle cerebral artery (P=0.002), and 1.1 (0.8) compared with -0.8 (0.8) for the right middle cerebral artery (P=0.002). There was no difference in the mean (S.D.) blood pressure increase resulting from the two agents: L-NMMA, 19.7 (7.4) mmHg; noradrenaline, 15.5 (4.8) mmHg (P=0.281). These results suggest that nitric oxide mediates at least part of the dynamic phase of cerebral autoregulation in humans. Reduced nitric oxide release may play a role in the impaired cerebral autoregulation seen in patients with, or at risk of, cerebral ischaemia.