The BOLD signal is an indirect hemodynamic signal which is sensitive to cerebral blood flow (CBF), cerebral blood volume (CBV) and oxygen extraction fraction (OEF). We observe in the anesthetized monkey a vasodilatory ceiling effect during inhalation of 6% CO2 where CBF or CBV is not further increased by an additional sensory stimulus. We show with simultaneous fMRI and electrophysiology recordings, that the fMRI response during 6% inhaled CO2 to a sensory stimulus reflects an oxygen extraction weighted signal. With this method, oxygen extraction fraction can be imaged by means of fMRI without injection of an exogenous drug. Combined electrophysiology and fMRI recording were performed in the primary visual cortex (V1) of 5 anesthetized monkeys in 8 experimental sessions using the techniques described by Logothetis et al in Nature 2001 [1,2]. Hypercapnia was induced by administration of premixed medical gases containing 3% or 6% CO2 and 21% O2. Ventilation with 3% and 6% CO2 increased end-tidal CO2 by approximately 9mmHg and 20mmHg, respectively. For visual stimulation, a rotating full field checkerboard was presented to both eyes of the monkey (12s on /12s off/ 39s on). For analysis of the BOLD data, a region-of-interest (ROI) has been selected at normocapnia restricted to V1 and correlation coefficients larger than 0.15. During normocapnia, the ROI in V1 contained in average 120±60 voxels with a positive correlation coefficient. During 3% CO2 the amplitude of the BOLD response decreased and only few voxels (5±5) were negatively correlated to the paradigm. During 6% CO2 most active voxels within the ROI (72±42) were negatively correlated. For time courses see figure 1. Importantly, the amplitude of the stimulus-induced responses of MUA and LFP responses during stimulation remained stable during both 3% and 6% CO2 compared to normocapnia. CBF or CBV acquired in different sessions did not respond to stimulation for 6% CO2. The temporal dynamics of the inverted BOLD response for 6