The quantity and quality of the language input that infants receive from their caregivers affects their future language abilities; however, it is unclear how variation in this input relates to preverbal brain circuitry. The current study investigated the relation between naturalistic language input and the functional connectivity of language networks in human infancy using resting-state functional magnetic resonance imaging (rsfMRI). We recorded the naturalistic language environments of 5- to 8-month-old male and female infants using the Linguistic Environment Analysis (LENA) system and measured the quantity and consistency of their exposure to adult words and adult–infant conversational turns. Infants completed an rsfMRI scan during natural sleep and we examined functional connectivity among regions of interest previously implicated in language comprehension, including the auditory cortex, the left inferior frontal gyrus (IFG), and the bilateral superior temporal gyrus (STG). Consistent with theory of the...