The dielectric properties of whole brain, skin, and skull were determined experimentally in the frequency range 300 KHz-300 MHz. Tissue samples were excised from 10, 30, and 70 day old Wistar strain rats. The data are presented in graphical format and compared to previously published data in the frequency range 0.1-20 GHz. Good agreement is observed between the two data sets. At frequencies in excess of about 100 MHz, where the g dispersion is dominant, the permittivity and conductivity increase monotonically with decreasing age. At lower frequencies, the site of the b dispersion, a change in the frequency dependence of the dielectric parameters is observed and is most evident in the spectra for brain and skin. This is attributed to changes in the tissue structure. Age-related dielectric data, available for 9 of the 34 tissues, were incorporated in a numerical plane wave exposure dosimetry study on anatomically heterogeneous rat models with body sizes corresponding to the ages of 10, 30, and 70 days at a number of spot frequencies from 27 to 2000 MHz. The results reveal that the variation in the dielectric properties affect the whole body SAR by less than 5% with the most conservative value (highest SAR) obtained when 70 day properties are used. Bioelectromagnetics Supplement 7:S12-S18, 2005.