This article discusses regeneration which is actually the dream of a scattering of biologists working in a relatively unheralded field called regenerative medicine. They hope to learn how other animals make whole their damaged parts, recreating complete working organs and appendages just as they did as embryos, but swiftly, and on a scale that matches an adult animal's dimensions. Growing whole new limbs will probably remain in the realm of science fiction for some time. But researchers harbor more immediate hope for replacing the muscle in damaged hearts. Although biologists have been fascinated by regeneration for at least 200 years, the signals that control the process have until recently remained a mystery. Within the past year, several groups have published new findings that suggest mammalian hearts could regenerate with the right sort of chemical nudge.