Manganese-enhanced MRI (MEMRI) is a powerful technique for imaging rodent neuroanatomy. We investigated differences in the uptake of paramagnetic manganese ions (Mn2+) in the mouse brain at two postnatal days: P7 and P21. We observed higher uptake of Mn2+ in all neuroanatomical regions-of-interest (ROIs) at P7; this increased uptake was reflected in the shorter T1 times at P7 relative to P21. We also observed faster clearance of Mn2+ within the ventricles at P21. Our findings highlight the spatiotemporal differences in Mn2+ uptake and may be useful when planning MEMRI studies in the developing mouse brain.