Studies of bioluminescence in living animals, such as cell-based biosensor applications, require measurement of light at different wavelengths, but accurate light measurement is impeded by absorption by tissues at wavelengths <600 nm. We present a novel approach to this problem—the use of a plastic window in the skin/body wall of mice—that permits measurements of light produced by bioluminescent cells transplanted into the kidney. The cells coexpressed firefly luciferase (FLuc), a vasopressin receptor—Renilla luciferase (RLuc) fusion protein, and a GFP2-β-arrestin2 fusion protein. Following coadministration of two luciferase substrates, native coelenterazine and luciferin, bioluminescence is measured via the window using fiber optics and a photon counter. Light emission from the two different luciferases, FLuc and RLuc, is readily distinguishable using appropriate optical filters. When coelenterazine 400a is administered, bioluminescence resonance energy transfer (BRET) occurs between the RLuc and GFP2 fusion proteins and is detected by the use of suitable filters. Following intraperitoneal injection of vasopressin, there is a marked increase in BRET. When rapid and accurate measurement of light from internal organs is required, rather than spatial imaging of bioluminescence, the combination of skin/body wall window and fiber optic light measurement will be advantageous.