To show the potential of these BRET systems for monitoring PPIs in deep tissues of small animals, we generated a bimolecular FRB/FKBP12 system of BRET6, which acts under the control of rapamycin. Our sensor represents a model of a drug-mediated association of two proteins, and we successfully imaged this PPI in deep tissues of mice. The BRET6 FRB/FKBP12 sensor noninvasively reports the rapamycin-induced interaction, which is shown by a significant increase of the A/D in the presence of rapamycin relative to control experiments. This effect is equally detectable in cultured cells and mice with similar DRs. Blocking the rapamycin effect in cells using the well-known inhibitor FK506, we showed, under cell culture conditions, that the observed ratiometric difference reflects a specific interaction between FRB and FKBP12.