Cortical spreading depression (CSD) has been implicated in the migraine aura and in stroke. This study demonstrates near-infrared spectroscopy (NIRS) for the first time as capable of noninvasive on-line detection of CSD in the pentobarbital-anesthetized rat. CSD was accompanied by a brief and rapid increase of regional CBF (by laser-Doppler flowmetry) to 200–400% baseline. NIRS demonstrates that this hyperperfusion is associated with concentration increases of oxyhemoglobin, while deoxyhemoglobin decreases. Simultaneously, oxygen partial pressure, measured on the brain surface with a solid-state Polarographic probe, was shown to be raised by at least 14 mm Hg during CSD. Oxygen-dependent phosphorescence life-time quenching measurements confirmed this finding. NIRS data on cytochrome aa₃, however, showed a CSD-related shift toward a more reduced state, despite raised blood oxygenation. This may suggest either limited O₂transport from the blood to mitochondria or decreased oxygen utilization during CSD as supposed by theories about compartmentalization of energy metabolism favoring glycolytic rather than aerobic energy supply during CSD. However, the data on cytochrome aa₃, warrant caution and are discussed critically. Nitric oxide synthase inhibition by systemic application of N′-nitro-l-arginine had no significant effect on the perfusion response or the tissue Po₂during CSD. During most CSD episodes, a brief decrease in MABP by 4–8 mm Hg was noted that might be caused by functional decortication during CSD.