An experiment described previously showed a large increase in weight and protein of the intestinal mucosa of suckling piglets during the first 24 h after birth. This did not take place in piglets that were starved. The results might have been partly due to the inclusion of protein molecules in the mucosa in process of absorption. Rabbits do not absorb large quantities of protein after birth, and the experiment has now been repeated on them. The gastrointestinal tract of suckled rabbits also grew rapidly in the first 24 h, but again not in those that were given only water, which is in line with the suggestion that colostrum contains a factor which stimulates the growth of the gastrointestinal tract. The brain gained weight in the suckled animals, due to incorporation of lipid and protein. It gained weight too in the starving rabbits, demonstrating its high priority for nutrients at a time when its growth velocity is at its peak.