Although, the first years of our life are influential in adulthood, we can barely remember detailed memories for events (`episodic memory`) from that time. The underlying reason may be the dramatic changes that the hippocampus undergoes postnatally. Episodic(-like) memory is also known to develop late in rodents. As such, hippocampal-dependent memory starts to emerge from ~3weeks of age. This time window is associated with changes in the molecular composition of neurons and the emergence of hippocampal activity mechanisms implicated in the mature memory (e.g. place cells, replay).In order to understand how these changes relate to each other transgenic animal models are required, which to date are mostly mice. However, experimental approaches to record neuronal activity from behaving mouse pups are limited.For this reason, we set up and successfully implemented chronic, extracellular electrophysiological recordings in freely moving mouse pups, performing a spatial learning and memory task. By adapting a tetrode system to the size and weight of a young mouse, we are now able to record hippocampal neuronal activity and test memory in the radial arm maze in pre-weaning animals. Analysis are underway to understand how changes in hippocampal single-neuron and network function relate to the maturation of long-term spatial memory.In the future, we plan to use this method to gain a deeper understanding of the interaction between molecular and system maturation by using transgenic mouse models. With this we hope to bridge molecular and systems neuroscience research.