In the paper ellipsoidal phantom model for investigation of energy absorbed by living organisms has been proposed. Calculations using the FDTD method of specific absorption rate (SAR [W/kg]) have been presented in the function of frequency, in the frequency band of 0,8–1,8 GHz, for plane wave and waveguide exposure conditions. Phantom model has been filled with liquid of electrical properties equivalent to electrical properties averaged over whole animal body. Values of the averaged electrical properties were established by measuring of the ε′ and ε″ of minced animal body. The validation of this phantom method has been done by measuring of SAR of living (anesthetized) mice in rectangular waveguides. The investigation of SAR of mice at different ages (10 day-old, 30 day-old and 4 month-old) demonstrates that young mice absorb the same energy or even lower as old (mature) animals.