We demonstrate the in vivo reconstruction of all fluorescence resonance energy transfer (FRET) parameters, including the nanometer donor-acceptor distance, in a mouse. The FRET chemical targets cancer cells, and on internalization, the acceptor is released, in lieu of a targeted anticancer drug in chemotherapy. Our method provides a new vehicle for studying disease by imaging FRET parameters in deep tissue. (C) 2010 Optical Society of America