The development of colon mucosa during postnatal growth was analyzed in male BD IX rats using histological and radioautographic methods. Newborn animals already exhibited clearly recognizable crypts of Lieberkühn. DNA synthesis activity predominated in the lower two thirds of those crypts, as it constantly did during subsequent growth and in adult colons. The height of crypts at birth was already half the adult size. There was, in contrast, a dramatic increase in their number (more than 1,000% from birth to the 10th week, as observed on transverse sections). The mechanism by which new glands derive from the mucosal epithelium implies a longitudinal fission of preexisting glands.