We assessed the impact of protein-calorie malnutrition on the developing rabbit intestine. Litters expanded 7 days postpartum (13-16 animals) were compared to control litters (6-8 animals). Chronic nutrient deprivation in early life markedly reduced small intestinal mass, as evidenced by depressed weight, crypt-villus length, protein content and DNA content. In kinetic studies, we observed impaired mucosal growth in undernourished rabbits, as demonstrated by depressed enterocyte proliferation and epithelial migration. Intestinal maturation also appeared to be modified, as suggested by altered mucosal enzyme activities and enhanced permeability which we evaluated by measuring unidirectional Na+ transport in short-circuited Ussing chambers. These findings may contribute to the increased severity of diarrheal illness observed in clinical malnutrition in infancy.