We think that the present work partly addresses the need for a carefully validated database of optical properties in rats to assist in the development and characterization of optical small animal imaging instruments and experiment design. The optical properties for heart, kidney, brain, and muscle and liver should be especially useful in the context of Monte Carlo simulations of optical photons traveling in biological tissues. Applications that could benefit from these data range from bioluminescence or tomographic optical fluorescence in preclinical models of cancer, to the control of the spatial extent of light pulse stimulations in recent optogenetic rodent models. Experimental measurements of tissues optical properties are scattered in the literature. Many studies focused on measurements at one or two wavelengths due to instrumental limitations (especially the lack of availability of short pulses laser sources) or targeted interest in a particular wavelength. As an example, the phototherapy research area is particularly active at evaluating optical properties in the red and near infrared.24 As a consequence, in the seminal review of biological tissues in optical properties,6 the vast majority of reported data were for red-infrared wavelengths for diffuse optical tomography and photodynamic therapy applications. From this first milestone, many studies have been carried out with a variety of techniques including reflectance and transmittance measurements with integral spheres, time resolved measurements,25 differential pathlength spectroscopy,26 confocal reflectance microscopy,27 and optical coherent imaging.28,29 Some techniques are limited to ex vivo imaging, whereas others lead to scattering properties only. A remarkable set of measurements for several organs in different species was carried out using the IAD method at four wavelengths in the 630- to 1064-nm range.23 Yet, when looking at the literature, few data are available, especially for rats, and no data are available for mice in the visible range, which correspond to the spectral window of excitation for most of the endogenous and exogenous fluorophores. As a consequence, researchers in the biophotonic field still use crude experimental measurement of maximum light transportation in tissue (see 30 for a discussion in the context of optogenetics) or extrapolate optical properties of human tissues to tailor small animal experiments.17 This arises from the intrinsic difficulty of measuring optical properties in living tissues and the limited dimensions of the rodent organs. The dynamic, in vivo, and spatially resolved measurement of optical properties in deep tissues still remains a challenging goal. Furthermore, absorption coefficients and reduced scattering coefficients from the literature show large variations up to several orders of magnitude for the same organs,6–8 and very few studies have been carried out for internal soft tissues such as heart, kidney, and liver. To our knowledge, apart from one study,23 no studies have reported data from several tissue types obtained in the same animals, with the same experimental set-up, making it difficult to interpret the large variability of optical coefficients seen in the literature.