Figure 1. Individual values for the effect of increased mean arterial pressure (MAP) on mean blood flow velocities in the middle cerebral artery (Vm). In the preclonidine group, initial MAP was increased 30 mmHg (+30 mmHg) by infusion of phenylephrine. After administration of clonidine, 5 micro gram/kg orally (postclonidine), MAP decreased; measurements of Vm were made and then repeated when the pressure had been restored to the preclonidine level (MAP-restored) and again when it had been increased a further 30 mmHg (+30 mmHg) by infusion of phenylephrine. Clonidine decreased Vm (P < 0.05). There was a statistically significant increase in Vm with a phenylephrine- induced 30-mmHg increase in MAP, before and after clonidine administration (P <0.05).