Acoustical outputs were determined for five ultrasonic generators, claimed by the manufacturers to be effective in driving rodents from buildings; wide variation of frequencies, amplitudes, and cycles, and pulse rates were found. Two contrasting types of generators were selected for field testing. One produces a wide band of frequencies, peaking at 41 kHz, but with other frequencies at varying amplitudes and pulse rates and giving a continuous output approaching white noise. The other produces a narrow frequency band, at approximately 22 kHz, approaching a pure tone; it gives intermittent signals of the highest amplitude of those tested. During the field testing Rattus norvegicus feeding activity was monitored and compared with the generators in an off- on sequence for 3-week periods in a room with the generators and an adjacent nonsound room; however, food consumption increased in the nonsound room. The narrow-band generator initially decreased rat feeding activity in the sound room and increased food consumption in the nonsound room. However, after 1 week, feeding activity increased again in the sound room, indicating habituation to the sound. It appears the aversive properties of these devices are not sufficient to keep rats from food.