Colonic and tail skin temperature of the unrestrained Fischer rat were measured immediately after a 90 min exposure to 600 MHz radiofrequency radiation in a waveguide-type system. Ambient temperature (Ta) was maintained at either 20, 28, or 35 C. The specific absorption rate (SAR) in dimensions of W/kg was controlled at a constant level through a feedback control circuit. The SAR needed to elevate colonic and tail skin temperature decreased with increasing Ta. For example, a 0.5 C elevation in colonic temperature occurred at SAR's of 4.3, 0.9, and 0.5 W/kg when Ta was maintained at 20, 28, and 35 C, respectively. Data from the present study were combined with data from earlier studies to assess the impact of varying Ta on the thermogenic effect of RF radiation in different species. In species ranging in mass from 0.02 to 3.2 kg, a double logarithmic plot of body mass versus SAR needed to elevate colonic temperature by 0.5 C was linear and inverse with a high goodness of fit (r(2) = -0.94). The highly correlated allometric relationship shows that, as body mass decreases, the relative impact of Ta on the thermogenic effect of RF radiation increases.