The absorbed dose rate in tissues can be calculated knowing the absorptivity, radiant energy fluence rate, and concentration of the photosensitizer. The propagation of visible light in malignant and nonmalignant tissues has been calculated by the discrete ordinates method for solving the Boltzmann transport equation, using absorption and scattering coefficients derived from measurements in tissues. A photodetector with isotropically responding fiberoptic probe imbedded in the tissue has been calibrated and used to measure the energy fluence rate in vivo. The effect of absorption in blood and in the photosensitizer dihematoporphyrin ether (DHE) has been investigated in a scattering medium or phantom. The generation and propagation of long wavelength fluorescence from absorption in DHE and in endogenous fluors is important in an apparatus designed to measure the concentration of DHE in tissues.