Introduction: Although much is known about how infants produce sounds during the first two years of life, little is known about how they change their speech behaviors during communication with another person. More research is needed to determine how infants alter vocalizations when communicating with a parent. This exploratory study focused on the length and complexity of infant vocalizations during mother-infant vocal interaction. The purpose was to examine how infant response type and maternal input influence the duration and number of syllables of infant vocalizations. Methods: Mother-infant communicative interaction was captured using high-quality digital audio recording. The duration and the number of syllables of each of the infants’ vocalizations were compared between two response conditions: (1) when the infant responded to the mother’s utterance, and (2) when the infant did not respond. The type of maternal utterance (comment, question, or command) was also coded to test for an effect on the vocalization variables. Results: Few trends emerged. There was large variability in median durations and their ranges across infants and conditions. Whereas the median number of syllables was similar across infants and conditions, the ranges of number of syllables differed. Discussion: The findings represent an important step toward identifying and categorizing infant speech behaviors and the ways in which mothers could increase speech development in their infants. The results of this study could serve as an additional component for describing infant speech development.