ULTRASONIC vocalisations commonly occur during social interactions among rodents. During mating, adult male and female rats (Rattus norvegicus) emit brief 50-60 kHz ultrasonic calls1; however, the function of these vocalisations is not known. In this study, we demonstrate that these mating calls have a precise function for communication. Specifically, 50-kHz vocalisations elicit sexual activity in female rats. Female rats exhibit a series of solicitation patterns during sexual behaviour, including orientation, darting and ear wiggling. These movements excite the male and enhance the likelihood of mounting, thereby facilitating copulation2-4. (Beach has emphasised the importance of solicitation by the female during mating and has suggested the term 'proceptivity' to include the female's behaviour in the initiation and maintenance of copulation.) Although the presence of an intact male is usually a prerequisite for solicitation behaviour, the specific sensory cues which elicit it are not known. Isolated oestrous female rats exposed to ultrasonic vocalisations from a male exhibited a shorter latency to, and higher rate of darting when subsequently placed in a mating situation5. Although the auditory cues primed oestrous females to display increased solicitation behaviour when later confronted with an intact male, there was no indication that these ultrasonic vocalisations had a direct function for communication in the induction of these behaviour patterns. The objective of the present report was to determine if ultrasonic mating calls of rats have a direct function for communication during copulation. © 1978 Nature Publishing Group.