Analysis of the spatiotemporal coupling between neuronal activity and cerebral blood flow requires the precise measurement of the dynamics of RBC flow in individual capillaries that irrigate activated neurons. Here, we use two-photon microscopy in vivo to image individual RBCs in glomerular capillaries in the rat dorsal olfactory bulb. We find that odor stimulation evokes capillary vascular responses that are odorant-and glomerulus-specific. These responses consist of increases as well as decreases in RBC flow, both resulting from independent changes in RBC velocity or linear density. Finally, measuring RBC flow with micrometer spatial resolution and millisecond temporal resolution, we demonstrate that, in olfactory bulb superficial layers, capillary vascular responses precisely outline regions of synaptic activation.N oninvasive imaging techniques have been widely used to determine the distributions of brain regions activated by sensory stimulations. Although several techniques detect contrast changes based on activity-dependent hemodynamic changes, the precise coupling between neuronal activation, energy demand, and changes in blood flow remains unclear (for review, see refs. 1-3). Analysis of the coupling requires a simultaneous measurement of the activity of neurons and the dynamics of local blood flow. Recently, measurements of extracellular activity combined with functional MRI (fMRI) have shown that field potential amplitude (4) and spiking frequency (5) are related to blood oxygen level-dependent (BOLD) signals (6, 7). These observations are a technical tour de force; yet, the spatial and temporal resolutions of fMRI are not sufficient to provide a precise understanding of the entire chain of cellular and vascular events elicited by the stimulation. In the cerebellar cortex, recent ''semiinvasive'' techniques requiring a craniotomy and using electrophysiological recordings combined with laser Doppler flowmetry have allowed more precise investigations and have demonstrated that the relationship between presynaptic and postsynaptic activity with blood flow varies according to the activated input (8-11). However, these studies did not measure blood flow in the individual capillaries that precisely irrigate the activated neurons, a prerequisite to analyzing the neurovascular coupling in detail.Two-photon microscopy imaging of blood flow represents an alternative approach to studying this coupling (12) in vivo. It allows measurements of RBC flow with micrometer spatial resolution and millisecond temporal resolution in individual capillaries and can be combined with electrophysiological recordings (13,14). If performed in a brain region where synaptic interactions are topographically organized, two-photon imaging should allow measurements of vascular flow in a given capillary irrigating a small population of neurons and glial cells. Because of its anatomical and functional organization, the rodent main olfactory bulb (15) is particularly suitable for such a study: all sensory cells expressing a given odorant r...