Ultrasonic calls used by rats during intraspecies communication have acoustic features, which can be selectively recognized by recipients, and possess a quantitative dimension reflecting the magnitude of the sender's response. This paper reviews basic principles of animal communication with particular attention to rat calls, and the features of ultrasonic calls, which could reflect such a quantitative aspect. Isolation calls of rat pups vary in frequency and duration and have changing sonographic structure over time. It is hypothesized that the quantitative "message" for the dam is encoded not only in the number of calls but also in frequency sweeps. The 22-kHz alarm calls of adults are characterized by a relatively constant sound frequency, marginal frequency modulation, and remarkable variability in call duration. It is hypothesized that quantitative aspect of these calls may be encoded in call length. Finally, the 50-kHz calls of adults, which are emitted in appetitive behavior are very short calls, with a relatively constant call duration, and a variable sound frequency. It is hypothesized that the peak frequency as well as the number of calls per time unit reflect the quantitative aspect in 50-kHz calls.