Functional MRI (fMRI) is a non-invasive technique widely used to map brain-functions. Nevertheless, its hemodynamic basis and spatial precision with which fMRI reflects sites of neuronal activity are not completely understood. We therefore combined fMRI, based on the blood oxygenation level dependent (BOLD) effect, with optical recording of intrinsic signals (ORIS), a technique, which has a better spatial and temporal resolution. Furthermore, ORIS can distinguish between localized changes in deoxyhemoglobin, and more widespread changes in cerebral blood volume/flow. In gerbils hemodynamic responses over the contralateral barrel cortex were studied with both methods, using identical stimulation of a single vibrissae and identical integration and correlation analysis strategies. Analysis of integration maps and of the spatial distribution and temporal correlation with the block-design of vibrissal stimulation revealed that the BOLD signal, at the site of neuronal activation, does not reflect a depletion of deoxyhemoglobin, as generally assumed. Instead, its positive polarity is likely due to an increase in cerebral blood volume (CBV) whose highly dynamic effect on the BOLD signal exceeds that of the increase in deoxyhemoglobin remaining elevated during prolonged stimulation. This is so, because we show, that blood flow does wash out deoxyhemoglobin but at a rate which is to decrease the deoxyhemoglobin concentration in the voxel below resting level. The wash out causes an accumulation of deoxyhemoglobin in the draining venous side, but at a time window which can be clearly distinguished from the specific activity by applying an analysis strategy based on correlation functions. Therefore, draining veins do not appear as confounding problem. This knowledge could be useful to model the BOLD effect more accurately and improve the spatial resolution of fMRI.