We have investigated the effect of an absorbing object on the time- course and the migration paths of photons within a highly scattering cylindrical phantom. Experimentally, we injected photons into the phantom at one point on the circumference, and recorded the time course of the photons arriving at various detection positions round the cylinder. The simulations used both a Monte-Carlo approach and a diffusion approach to calculate the photon migration. The two computational approaches are similar. The calculated time-course signals agree well with the experimentally observed signals. Moreover, we are able to use a diffusion approximation to calculate the probable paths taken for photons which take a defined time to travel from source to detector.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.