In contrast to observations in other species, neither enteric mucosal weight, protein content, nor DNA content of either the suckled or artificially fed guinea pig changes over the first 24 h following term birth. In guinea pigs delivered 5 days prematurely, however, statistically significant increases in proximal enteric mucosal mass (38%), protein content (51%), and DNA content (35%) occur in suckled animals over the first 24 h of life but not in artificially fed animals. These observations illustrate that the enteric mucosal hyperplasia secondary to natural feeding, quantitatively and temporally, is species-specific and suggest that this effect is manifested only during a finite period of intestinal development. They raise doubts, therefore, concerning the importance of early enteric mucosal hyperplasia in adaptation of all species to extrauterine life.