Understanding of redox signaling requires data on the spatiotemporal distribution of hydrogen peroxide (H2O2) within the cell. The fluorescent reporter HyPer is a powerful instrument for H2O 2 imaging. However, rapid diffusion of HyPer throughout the nucleocytoplasmic compartment does not allow visualization of H 2O2 gradients on the micrometer scale. Here we dramatically improved the spatial resolution of H2O2 imaging by applying subcytoplasmic targeting of HyPer. The membrane-attached reporters identified "microdomains" of elevated H2O 2 levels within the cytoplasm of the cells exposed to growth factors. We demonstrate that diffusion of H2O2 across the cytoplasm was strongly limited, providing evidence that H2O 2 acts locally inside cells.