Studies on animals with gliomas inoculated in the brain and treated with intratumoral injections of 201Tl-chloride have previously shown very promising results, with a survival several weeks longer than controls. Total regression was found in some animals, and necrosis was found in all the 201Tl-treated brain tumors. This study was undertaken to estimate the absorbed dose and dose distribution to the tumor based on the localization and clearance properties obtained from images with two high-resolution imaging techniques; pinhole single photon emission computed tomography (SPECT) and beta-camera. The images from the beta-camera were used to calculate the absorbed dose rate, using an in-house-developed, voxel-based Monte Carlo program, based on the EGS4 package. To evaluate the effects of different beta-particle energies on the absorbed dose rate distribution, simulations of medium- and high-energy electrons were conducted. Dose-volume histograms from these simulations show that the energy absorption is very locally distributed for 201Tl and medium energy, whereas high-energy beta emitters show a broader dose-volume distribution. The calculated total absorbed dose of 2-8 Gy in the tumor seems to be relativity low when considering the therapeutic effect that was seen. Further investigations, to determine the cause of the high therapeutic efficacy, are needed.