Introduction: Previous studies underscore the importance of speech input, particularly infant-directed speech (IDS) during one-on-one (1:1) parent–infant interaction, for child language development. We hypothesize that infants’ attention to speech input, specifically IDS, supports language acquisition. In infants, attention and orienting responses are associated with heart rate deceleration. We examined whether individual differences in infants’ heart rate measured during 1:1 mother–infant interaction is related to speech input and later language development scores in a longitudinal study.