Ultrasonic calls are emitted by many species of rodents in a variety of situations. In particular, infants commonly emit such calls when removed from the nest; the rate and intensity of calling are related to the degree of development of homoiothermy. The relevant biological significance of these signals is documented by their capability to promote parental behaviour, such as maternal retrieval. There is recent evidence that ultrasonic vocalization in rodent pups could be valuable as a bioassay in Behavioural Toxicology. In particular, the results of our recent studies together with those of other authors suggest that ultrasonic calls emitted by infant rats could be considered a useful test in detecting subtle effects of adverse treatment during development.