1. Measurements of intracellular methionine concentration in pig colon, following in vitro incubation in methionine containine medium, have been carried out during the first 10 days of post-natal life. 2. The new-born pig colon concentrates methionine within its mucosa. Autoradiography of 3H-labelled phenylalanine shows it to be accumulated to greatest extent in cells located in the surface epithelium. 3. The ability of the colonic mucosa to concentrate methionine disappears during the first few days of post-natal life. 4. Radioactive thymidine, injected at birth, shows cell mitosis to be confined to the lower third of each colonic crypt. Labelled cells later migrate out of the crypts on to the surface epithelium. The cell replacement time is 105 h. 5. The time course for the fall in the ability of the pig colon to actively accumulate methionine corresponds to that predicted if cells synthesized from birth were unable to transport this amino acid. 6. The ability of the new-born pig colon to transport amino acids, though transient, may be physiologically important. The nature of the signal which changes the physiological function of cells synthesized post-natally remains to be determined.