1. 1. Colonic and tail skin temperature of the unrestrained Fischer rat were measured immediately after a 90 min exposure to 600MHz radiofrequency radiation in a waveguide-type system. Ambient temperature ( T a) 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. 2. 2. The SAR needed to elevate colonie and tail skin temperature decreased with increasing T a. For example, a 0.5°C elevation in colonic temperature occurred at SARs of 4.3, 0.9 and 0.5 W/kg when T a was maintained at 20, 28 and 35°C, respectively. 3. 3. Data from the present study were combined with data from earlier studies to assess the impact of varying T a 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 colonie temperature by 0.5°C was linear and inverse with a high goodness of fit ( r 2 = − 0.94). 4. 4. The highly correlated allometric relationship shows that, as body mass decreases, the relative impact of 7, on the thermogenic effect of RF radiation increases.