A total of 48 male infants experienced either conversational turn-taking or random responsiveness of their mothers when aged 0;3 and 0;4. In both periods, the infant's rate of vocalizing was not significantly influenced by the contingency of the mother's response, but contingency altered the temporal parameters of the infant's vocal pattern. Infants tended to produce more bursts or packets of vocalizations when the mother talked to the infant in a random pattern. When the infants were aged 0;3 such bursts occurred most often at intervals of 0.5-1.5 sec whereas when they were aged 0;4 they took place most frequently at significantly longer intervals, of 1.0-2.0. The difference corresponded to the difference between intervals with which the mother responded contingently to vocalizations of the infant at 0;3 and at 0;4. While the intervals (between the onset of the infant's vocalization and the onset of the mother's vocalization) rarely exceeded 0.5 sec when the infant was aged 0;3, they were mostly distributed between 0.5 and 1.0 sec when he was aged 0;4. After vocalizing spontaneously, the infant tended to pause as if to listen for a possible vocal response from the mother. In the absence of a response, he vocalized repeatedly. The intervals between the two consecutive vocalizations were changed flexibly by the infant according to his recent experience of turn-taking with the mother.