Mice emit ultrasounds (70 ± 5 kHz) in infancy as well as in adulthood. Both male and female NMRI mice emit ultrasounds during the first 3 minutes of presentation of a conspecific female. The number of calls recorded is higher in female-female than in male-female pairs. If the same female is represented after a short interval of time to the conspecific the amount of calls uttered decreases. This suggests that ultrasounds can be used as an index of social memory. The time interval necessary to determine this decrease, following successive exposure to the same female, is different in male and female mice. In contrast to males, females show lower ultrasonic emission when the same female is reintroduced in their home cage four hours after the first exposure. In contrast, these animals emit a similar amount of calls when a different female is presented after the same time interval. If 24 hrs elapsed from the first presentation, the ultrasonic performance of males and females does not change according to the familiarity with the female partner. This suggests that females are able to recognise a female conspecific after a four-hour time interval, whereas males' ultrasonic performance is not affected by the characteristics of the female partner. A detailed behavioural analysis of the first minutes of male-female and female-female interaction was conducted to ascertain if different motivational systems are involved according to the sex of the vocalising animals.