Whole-body evaporative heat loss was measured as whole-body evaporative water loss in mice during a 90 min exposure to 2450-MHz microwave radiation at an ambient temperature of 20 C and in non-exposed mice maintained at ambient temperature of 0, 25, 30, 33, and 35 C. The ambient-temperature threshold for increasing evaporative water loss was between 30 and 33 C. A specific absorption rate of microwave radiation in excess of 29 W/kg was required to produce an increase in heat loss. For absorption rates ranging from 29 to 44 W/kg, the mouse dissipated 65% of the total absorbed heat by water evaporation; the remainder was dissipated passively. The data collected in the mouse may be extrapolated to larger species, such as man, but only by an exponential relationship. Using this relationship, it was shown that a threshold specific absorption rate of 29 W/kg in a 0.033-kg mouse was equivalent to approximately 0.25 W/kg in a 70-kg human.