The attenuation of light in rabbit liver and striated muscle and in pig brain has been measured in the wavelength range400–800 nrn. A variable‐wavelength monochromatic light source was used to illuminate the surface of the exposed tissues in situ. Optical fibers, coupled to a single photon counter, were positioned within the tissues at different depths to determine the distribution of light flux along the central axis of the light beam. The wavelength‐dependence of attenuation was measured in vivo, and changes occurring in the attenuation spectra immediately post mortem were recorded. Strong absorption bands were observed around 425 nm and 550 nm in all three tissues, both in vivo and post mortem. At longer wavelengths, the attenuation decreased slowly, and no significant structure was observed. Substantial changes occurred immediately post mortem, particularly in the absorption bands. These effects are attributed mainly to de‐oxygenation of blood, and changes in blood concentration in the tissues. The implications for light dosimetry in photodynamic therapy are discussed.