Rats spent more time in the halves of shuttle boxes that were shielded from illumination by 1.2 GHz microwave energy than in the unshielded. In Experiment 1, rats avoided the energy when it was presented as 30-musec pulses with a pulse repetition rate of 100 pulses per second (pps). The average power density was about .6 mW/cm2, and the peak power density was about 200 mW/cm2. In Experiment 2, the energy was presented both continuously and in pulse-modulated form, i.e., .5-msec exponentially decaying pulses at a rate of 1,000 pps. The average power density of the continuous energy was 2.4 mW/cm2, and the average power density of the pulse-modulated energy was .2 mW/cm2. The peak power density of the modulated energy was 2.1 mW/cm2. The rats avoided the pulsed energy, but not the continuous energy.