Early postnatal administration of SCH 23390 and sulpiride induced marked changes in the ultrasonic vocalization elicited by the removal of rat pups from their nest. In particular, SCH 23390 produced a significant increase in the length, as well as in the sound pressure level, of ultrasonic calls; moreover, a significant decrease in minimum and maximum frequency values was found in pups treated with this DA1-receptor antagonist. Sulpiride significantly reduced the rate of calling, as well as the pressure level of sounds, whereas it did not influence other parameters of the ultrasonic emission. These behavioural alterations seemed to be the consequence of an impaired functional maturation of the dopamine (DA) system; however, the different changes caused by SCH 23390 and sulpiride, respectively, suggest that DA1- and DA2-receptor populations could play a distinct role in the ultrasonic calling during early postnatal life.