The first days after birth in ferrets provide a unique view of the development of a complex brain. Unlike mice, ferrets develop a rich pattern of deep neocortical folds and cortico-cortical connections. Unlike humans and other primates, whose brains are well differentiated and folded at birth, ferrets are born with a very immature and completely smooth neocortex: folds, neocortical regionalisation and cortico-cortical connectivity develop in ferrets during the first days after birth. After a period of fast neocortical expansion, during which brain volume increases by up to a factor of 4 in 2 weeks, the ferret brain reaches its adult volume at about 6 weeks of age. This dataset contains brain MRI T2 data from 28 ferrets from P0 to Adults. It can be visualised at http://brainbox.pasteur.fr/project/FIIND.