Easily detectable (5%-20%) transient increases in the intensity of water proton magnetic resonance (MR) signals in human primary visual cortex were observed during visual stimulation in gradient echo images at 4-T field strength. The signal intensity increases were predominantly restricted to areas containing gray matter and were used to produce high-spatial-resolution human functional brain maps. Time dependence of the functional brain maps also was monitored during visual stimulation using images acquired every approximately 5 seconds; these images with high spatial and temporal resolution demonstrated that photic stimulation first resulted in signal increases in a large area of the visual cortex followed by a reduction in the size of the area, and that signal intensity increases in the gray matter were time dependent. Reducing the image acquisition echo times reduced the amplitude of the fractional signal change, suggesting that it is produced by a change in T2 or T2*. The amplitude, sign, and echo time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral with the idea that neural activation increases regional cerebral blood flow (rCBF) with a concomitant increase in venous blood oxygenation.