Early postnatal development is a critical period for the establishment of functional brain networks. While extensive research has focused on local circuit maturation, the emergence and evolution of long-range communication between brain regions has received less attention. Here we investigate the developmental trajectory of inter-regional communication in the mouse brain over the first two postnatal weeks of life, a period characterised by rapid synaptogenesis followed by activity-dependent pruning and circuit refinement. We leverage electrophysiological datasets of population activity simultaneously recorded in pairs of cortical and subcortical brain regions in mice between postnatal days P2 and P12. We then employ multivariate statistical methods to quantify the strength and directionality of communication between regions over time. We find that different pairs of regions show distinct trajectories of inter-regional communication over the course of development. Notably, we observe a strong increase in communication strength between prefrontal regions and the subcortical structures of striatum and thalamus, consistent with the maturation of the cortico-striatal-thalamic loop during this time. While most pairs of regions exhibit bidirectional information flow, we observe predominantly feedback communication from the anterior cingulate to mediodorsal thalamus during the first postnatal week. When analysing the communication axes - unique patterns of neural activity most correlated between brain regions - we find differences in their stability across areas and across development, with subcortical-to-cortical communication appearing notably more dynamic than cortico-cortical communication. Overall, our approach shows promise in understanding the developmental dynamics of brain-wide communication, a topic that has been until now particularly understudied.