Sentences are the primary means by which people communicate information. The information conveyed by a sentence depends on how that sentence relates to what is already known. We conducted an fMRI study to determine how the brain establishes and retains this information. We embedded sentences in contexts that rendered them more or less informative and assessed which functional networks were associated with comprehension of these sentences and with memory for their content. We identified two such networks: A frontotemporal network, previously associated with working memory and language processing, showed greater activity when sentences were informative. Independently, greater activity in this network predicted subsequent memory for sentence content. In a separate network, previously associated with resting-state processes and generation of internal thoughts, greater neural activity predicted subsequent memory for informative sentences but also predicted subsequent forgetting for less-informative sentences. These results indicate that in the brain, establishing the information conveyed by a sentence, that is, its contextually based meaning, involves two dissociable networks, both of which are related to processing of sentence meaning and its encoding to memory.