The cerebrovascular response to hypercapnia and hyperventilation was studied in normal and jaundiced baboons by the intracarotid133Xe injection technique. The baboons with bile duct ligation were found to have decreased CBF at all levels of Paco2. This difference between normal and jaundiced baboons was 13% at normocapnia rising to 33% with hypercapnia and 37% with hypocapnia. The CBF values all were increased toward normal by use of an alpha-adrenoreceptor blockade (phentolamine). It is suggested that the obstructive jaundice potentiated an inherent vasoconstrictor alpha-adrenergic mechanism to oppose the effects of CO;. Also, alteration of the Paco2may have produced its effects on the cerebral vessels by altering this adrenergic mechanism.