This study investigates the role of wavelength in determining the source and dynamic range of activity-driven reflectance changes in macaque striate cortex. By using short (600 nm) and long (720 nm) wavelengths to map ocular dominance, orientation, and position from the same region of cortex on alternate trials, we isolated wavelength-dependent differences in the contributions of different tissue compartments. In agreement with previous reports, 600-nm illumination was found to produce optical signals that were more than twice the size of those obtained with 720-nm illumination. In addition, 600- and 720-nm images were found to correlate everywhere except in regions occluded by blood vessels, where the images obtained at 600 nm correlated with the overlying vasculature. Since the 720-nm images do not correlate with the vasculature, this difference suggests that differential images obtained under 600-nm illumination are disproportionately sensitive to vascular events (e.g., changes in blood flow, volume, etc.). This finding is supported by the absorption spectra of hemoglobin and its derivatives, which absorb 600-nm light 4-1000 times more strongly than 720-nm light. Hence, for the 40% of cortex covered by blood vessels larger than 50 microns, images obtained at 600 nm are dominated by the vascular compartment to the exclusion of signals from the neural compartment below.