Ultrasounds are emitted by mice in different social contexts. The main interest has been directed towards ultrasounds emitted by pups during their development that seem to affect mother-infant relationship. They have been used as a measure of stress that can be easily modulated pharmacologically. Adult mice emit ultrasounds during the first minutes of interaction with a potential sexual partner. Mainly the male seems responsible for these vocalisations, and data from the literature suggest that these calls represent a measure of male sexual motivation. Several experiments have been conducted to verify this hypothesis. NMRI outbred albino mice were tested for ultrasonic emission during the first three minutes of sexual encounters. Ultrasounds (70 + 5 kHz) were counted by the use of a bat detector (QMC Instruments). Female characteristics (strain, weight, estrous condition) did not seem to affect male ultrasonic emission. Genetic and experiential characteristics of the males did not represent variables affecting, from a quantitative point of view, males' ultrasonic calls. We did not find significant correlation between ultrasonic vocalisation (UV) and latency of ejaculation nor between male behaviours indicating strong arousal and UV. On the contrary we found a strong inverse relationship between aggressiveness and ultrasonic vocalisation: males with little UV towards females showed more male-male aggressiveness than males with high vocalisations. The amount of ultrasonic calls seemed rather affected by the test cage while not the behaviours: testing the male in the presence of the odour of another male sensibly decreased IJV. Moreover, when females were given the opportunity to choose between beddings of males with low and high levels of UV, diestrous females preferred the bedding of high vocalizing males while estrous females did not show clear preference. All together these data raised doubts about the causal and functional implications related to ultrasonic communication during the first minutes of sexual interaction.