Understanding of redox signaling requires data on the spatiotemporal distribution of hydrogen peroxide (H(2)O(2)) within the cell. The fluorescent reporter HyPer is a powerful instrument for H(2)O(2) imaging. However, rapid diffusion of HyPer throughout the nucleocytoplasmic compartment does not allow visualization of H(2)O(2) gradients on the micrometer scale. Here we dramatically improved the spatial resolution of H(2)O(2) imaging by applying subcytoplasmic targeting of HyPer. The membrane-attached reporters identified "microdomains" of elevated H(2)O(2) levels within the cytoplasm of the cells exposed to growth factors. We demonstrate that diffusion of H(2)O(2) across the cytoplasm was strongly limited, providing evidence that H(2)O(2) acts locally inside cells.