Components of the communication system between infant and adult rodents based on ultrasonic vocalizations (USVs) of infants are analyzed. USVs are most often emitted from a pup lost outside the nest in response to changes of: (i) body temperature, (ii) contact with adults/littermates, (iii) handling, and (iv) smell. These changes modulate the state of arousal and the emotional/motivational states and, as a result, USVs are produced. Acoustic properties of USVs seem to reflect the degrees of changes in arousal and emotion/motivation. Adult rodents are aroused by perceiving the USVs, locate the sender and show a phonotaxic approach to the sender. Acoustic properties of USVs in the frequency and time domains are described based on which adult rodents discriminate the USVs from other ultrasounds and take the USVs or adequate models of them as preferred goals of their phonotaxic approach. The preferred approach to adequate USVs is modulated by emotions/motivations, the sex of the receiver, hormonal states, experience with pups and neurotransmitter systems of the brain. The phonotaxis can be understood as the appetitive component of a pup-caring instinct. The consummatory act of the instinct is the retrieval of the lost pup. This retrieval is independent of USV presence, but it closes the communication loop activated by the emission of USVs. Communication with USVs can be used as a tool to investigate genetic and brain mechanisms of behavioral control.