Whole-body absorption of 2450-MHz radiation was measured in rats that ranged from 6 to 440 grams and mice that ranged from 30 to 50 grams. Simultaneous exposure of groups of animals in varying numbers and various configurations were made under free-field conditions in an electrically anechoic chamber. Measurements of whole-body absorption were made with twin-well calorimeters. The results indicate that, for animals within a given narrow range of body mass, individual rates of absorption may, at the extreme, vary by a factor of seven. One implication of this variability is that studies of dose-response relations may be confounded unless power densities of incident radiation are sufficiently different to provide nonoverlapping dose rates. Comparisons of measured doses to predicted values as derived from a spherical shell model show considerable variations of observed from predicted values. Specific relationships of the specific absorption rates (SAR) to position and to the position and mass of the animals are presented.