Cerebral blood flow (CBF) and cerebral venous blood gases were investigated in seven awake patients during hypotension induced with sodium nitroprusside. Infusion of the drug into the internal carotid artery produced no changes in cerebral haemodynamics. Intravenous nitroprusside infusion resulted in a decrease in both CBF and jugular venous PO2 (P less than 0.01, respectively). Reducing mean arterial blood pressure by 18% and 43% produced a mean CBF fall of 6% and 27% respectively. Arteriovenous oxygen difference increased in consonance with the CBF decrease (P less than 0.01). Arterial PCO2 fell during the infusion (P less than 0.01). At 18% blood pressure reduction, the observed PCO2 decrease of 0.25 kPa could account for the 6% CBF decrease. At 43% blood pressure reduction, PCO2 fell by 0.53 kPa, a decrease which could not explain the observed CBF fall of 27%. Dizziness and incipient fainting occurred in the patients at this point. It is concluded that sodium nitroprusside has a minor but significant effect on cerebral haemodynamics in normal awake man.