Altricial rodent pups emit ultrasonic vocalizations (USVs), which are whistle-like sounds with frequencies between 30 and 90 kHz. These signals play an important communicative role in mother-offspring interaction because they elicit in the dam a prompt response as concerning care-giving behaviors. To investigate neurobehavioral development, the analysis of the number of USVs presents several advantages: (1) USVs are one of the few responses produced by very young rodents that can be quantitatively analyzed and elicited by quantifiable stimuli; (2) USV emission follows a clear ontogenetic profile from birth to the second to third week of life, thus allowing longitudinal analysis during very early post-natal ontogeny. The reported role played by several receptor agonists and antagonists in modulating the USV rate makes this measure highly informative in investigating the effects of toxicants and, more generally, psychoactive compounds on the development of selected brain systems.