Cerebral blood flow (CBF) is highly regulated by changes in arterial PCO₂ and arterial PO₂. Evidence from animal studies indicates that various vasoactive factors, including release of noradrenaline, endothelin, adrenomedullin, C-natriuretic peptide (CNP) and nitric oxide (NO), may play a role in arterial blood gas-induced alterations in CBF. For the first time, we directly quantified exchange of these vasoactive factors across the human brain. We measured CBF (Fick principle and transcranial Doppler ultrasound) in 12 healthy humans at rest and during hypercapnia (4, 8% CO₂), hypocapnia (voluntary hyperventilation) and during hypoxia (12, 10% O₂). At each level, blood was sampled simultaneously from the internal-jugular vein (v) and radial artery (a). With the exception of CNP and NO, the simultaneous quantification of noradrenaline, endothelin or adrenomedullin showed no cerebral uptake or release during changes in arterial blood gases. Hypercapnia, but not hypocapnia, increased CBF and caused a net cerebral release in nitrite (as a marker of NO), reflected by an increase in the v-a difference for nitrite [57±18 µmol/L (4% CO₂) and 150±36 µmol/L (8% CO₂); both P<0.05]. There was also release in cerebral CNP during changes in CO₂ (hypercapnia vs. hypocapnia; P<0.05). During hypoxia, there was a net cerebral uptake in nitrite which was reflected in a decreased v-a difference for nitrite [-96±14 µmol/L (10% O₂; P<0.05)]. These data indicate that there is a differential exchange of NO across the brain during hypercapnia and hypoxia and that CNP may play a complimentary role in CO₂-induced CBF changes.