The US National Toxicology Program (NTP) conducted a long-term carcinogenic and toxicity study with small animals on the effects of mobile phone radiation at 900 MHz and 1900 MHz, respectively, and showed an increase in incidence of malignant schwannomas in the heart of male rats at very higher exposure levels. To verify and clarify the NTP study results, Japan and Korea are conducting a joint study using the same reverberation chamber type exposure system as in the NTP study. The purpose of this paper is to derive a quantitative relationship between the electric field strength in the reverberation chamber and the exposure level, i.e., the mean whole-body averaged specific absorption rate (WBA _ SAR) of rats based on a large-scale finite difference time domain (FDTD) simulation. First, two FDTD simulation methods, one with a single plane wave incidence and the other with simultaneous plane wave incidence, were compared, and then the former method was adopted for convenience and saving calculation time. The derived quantitative relationship between the WBA _ SAR and the electric field strength required to achieve the WBA _ SAR consists of a one-dimensional approximation model which considers only the body mass of rats and a twodimensional approximation model which considers not only the body mass but also the number of rats. The two approximation models cover the entire two-year exposure period, and the exposure level in the two-year long-term exposure experiment is being well controlled. Moreover, unlike dosimetry in the NTP study, an uncertainty analysis of numerical dosimetry was performed in more details, and the combined standard uncertainty in the WBA _ SAR was found to be 16.1%.