Rates were exposed for short durations (30, 60, 120, and 240 sec) to electromagnetic irradiation at a frequency of 3000 MHz. At each duration of exposure percent mortality increased with increases in incident energy density (power density × duration). The incident energy density required to kill 50% ( LD 50 ) increased monotonically with increasing exposure durations. Lethal power density ( LD 50 ) was a decreasing monotonic function of exposure duration. In addition, body weight was found to be a factor in determining mortality.