Subarachnoid hemorrhage (SAH) was induced in baboons by puncturing the middle cerebral artery. Four to seven days later cerebral blood flow (CBF) responses to changing PaCO2, and to intracarotid infusion of 1.0, 2.5 and S.O µg of 5-hydroxytryptamine (5-HT)/kg/min were studied using the intracarotid xenon clearance technique. Indices of cerebral metabolism were determined by measuring arterio-venous differences for oxygen, pyruvate, lactate and glucose. The results were compared with those from shamoperated baboons. In the sham-operated group normal O2 reactivity was seen, and 5-HT infusion did not produce any significant change in CBF or cerebral metabolism. By contrast, the group in which SAH was induced showed a significant decrease in CBF and cerebral oxygen utilization, and attenuated O2 reactivity.