Live-cell detection of intracellular enzyme activity requires that substrates are cell-permeable and that the generated products are easily detected and retained in cells. Our objective was to create a novel fluorogenic substrate that could be used for real-time detection of apoptosis in living cells. We have synthesized a highly cell-permeable caspase-3 substrate, DEVD-NucView488, by linking a fluorogenic DNA-binding dye to the caspase-3 recognition sequence that renders the dye nonfunctional. On substrate cleavage, the dye is released and becomes highly fluorescent on binding to DNA. DEVD-NucView488 detected caspase-3 activation within a live-cell population much earlier and with higher sensitivity compared with other apoptosis reagents that are currently available. Furthermore, cells incubated with DEVD-NucView488 exhibited no toxicity and normal apoptotic progression. DEVD-NucView488 is an ideal substrate for kinetic studies of caspase-3 activation because it detects caspase-3 activity in real-time and also efficiently labels DNA in nuclei of caspase-3-activated cells for real-time fluorescent visualization of apoptotic morphology. The strategy utilized in the design of this fluorogenic substrate can be applied in future endeavors to develop substrates for detecting real-time intracellular enzyme activity.