Preterm infants' behavioral state and physiological parameters are affected by environmental noise and adult voices. Only a handful of studies have explored the effects of direct maternal vocal communication on preterm infants' autonomous nervous system responses. Furthermore, to our knowledge, no study to date has investigated the effect of the father's voice on preterm infant's behaviors and physiological parameters. This study evaluated the effects of both mothers' and fathers' infant-directed speech on preterm infants' behavioral states. Fourteen stable, premature infants serving as their own controls were videotaped while their mother and father were speaking to them for 5 min over 2 consecutive days. Infants' behavioral states and state lability were coded for each voice presentation (father and mother), in the three different conditions, before, during, and after the intervention. Present results show an interaction between vocal intervention and infant behavioral state. Both maternal and paternal speech modified infant behavioral state, but no significant difference in the behavioral state distribution was observed between mother's and father's voice presentation. Infants spent more time in a quiet alert state when they heard both voices compared to no vocalization baseline. These findings indicate the importance of both the fathers' and the mothers' voice for preterm infants. The parental vocal intervention has an awakening effect. Further studies are needed to better identify the benefits for preterm infants of a relational care approach.