The reconstruction of fluorescence lifetime distributions in heterogeneous turbid media and tumor-bearing animals are experimentally demonstrated by frequency-domain measurements. A set of coupled diffusion equations are used to describe the propagation of excitation and fluorescent emission light in multiply scattering media. A finite element based reconstruction algorithm combined with Marquardt and Tikhonov regularization methods are used to obtain the fluorescence images. The experimental set-up is an automatic multi-channel frequency-domain system. 16 sources and 16 detectors are used. Experiments are performed using indocyanine green (ICG) and 3,3'-diethylthiatricarbocyanine iodide (DTTCI) in tissue-like phantoms of both single- and multi-target configurations with considerations of perfect and imperfect uptake of fluorescence dyes in the scattering media. ICG are used in tumor-bearing animal studies. Our results show that the fluorescence lifetime image of the heterogeneities within a circular surrounding medium and in-vivo tissue can be reconstructed successfully.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.