The impact of early postnatal protein-energy malnutrition and of 4, 7, and 14 d of nutritional rehabilitation on small intestinal growth, development, structure and function was examined in 28-, 32-, 35-, and 42-d-old infant rabbits. Malnutrition was induced by litter expansion 7 d postpartum and, in randomly selected malnourished animals, refeeding was begun at weaning, 28 d. Results are compared toad libitumfed dietary controls. Malnutrition altered the small intestine of the developing rabbit, as evidenced by: 1) reduced jejunal and ileal mass as shown by decreased mucosal wt, protein, and DNA content; 2) depressed epithelial proliferation and enterocyte migration along the crypt-villus axis; 3) delayed epithelial maturation as measured by mucosal enzyme activities; and 4) enhanced glucose-stimulated Na+transport. Refeeding stimulated rapid and complete recovery, as evidenced by: 1) restoration of jejunal and ileal mucosal mass within 4 d; 2) enhancement of epithelial renewal and enterocyte migration by 7 d; and 3) complete return of the normal pattern of mucosal enzymes by 14 d. With 7 d of refeeding, glucose-stimulated Na+transport was downregulated to the level of dietary controls. We conclude that early postnatal protein-energy malnutrition has a severe impact on small intestinal growth, development, structure, and function. Furthermore, a brief period refeeding induced a rapid and complete recovery of these parameters.