The ability to lay down detailed and specific hippocampal-dependent memories and the capability to retain these over extended periods of time develops gradually . In mice, adult-like hippocampal-dependent memory starts to emerges ~3 weeks of age. Spatial representations (place, grid head direction cells) emerge with a similar timeline and so does memory replay, the spontaneous repetition of experience-related activity patterns. Memory replay is thought to be key to information communication between the hippocampus and neocortex. It is therefore important to assess the developmental timeline of hippocampal cortical interaction.To this end, we set up optical (wide-field) voltage imaging of dorsal cortices along with laminarly-resolved hippocampal electrophysiology in sleeping 3- to 4-week-old mouse pups, to get a large-scale view of the brain activity patterns observed during the period when critical developmental milestones for hippocampal memory occur.Previous work from our labs in adult rodents has characterized importance of the bidirectional communication of the hippocampus and cortical area, especially the retrosplenial cortex and associated cortical networks during sleep. Here, we set out to characterize how these networks emerge in development. To date, we have successfully set up the method in mouse pups and are able to record sleep and rest in pre-weaning head-fixed mouse pups reliably. Analyses are underway to chart the emergence of mature hippocampal-cortical networks.