Instead of creating rodent-proof structures, electric devices emitting high-frequency sounds are installed in order to drive out already established rodents, or discourage others from invading the premises. Rodents communicate to a considerable degree at frequences around 40 to 50 kHz, occasionally up to 100 kHz, and rats are known to produce ultrasounds as a method of echolocation. Tests carried out at the Danish Pest Infestation Laboratory with 11 different devices, including such with varying frequencies and random intervals between signals, all indicate that apart from an initial repellency lasting from 30 min to 3 hr, no durable influence on rat behavior can be achieved. If ultrasound devices are as harmful to rodents as claimed, they are likely to be so also to other mammals including man, pets, and domestic animals. Rodents usually feed in the particular hidden areas where they feel safe, and the ultrasound devices in no way can change the situation.