The effect of age, weaning and postweaning diet on small intestinal growth and morphology were investigated in young swine. Small intestine weight and villus height, measured at the midpoint of the small intestine (i.e., jejunum), were determined in suckling and weaned pigs. Scanning electron microscopy was performed on jejunal specimens from suckling pigs killed at 2, 10, 21, 28 and 35 d of age and in 21-d and 35-d weaned pigs at various ages postweaning. A 2 X 2 factorial arrangement of postweaning diets also was used to investigate jejunal morphological measurements in a 21-d-old weanling pig group. These dietary treatments evaluated the effects of 0 or 25% added dried whey and 0 or 6% added corn oil. The morphology results demonstrated that jejunal villus height declined during the suckling period, with a marked reduction at 3 and 7 d postweaning for both 21-d-old and 35-d-old weaned pigs. Transmission electron microscopy also demonstrated long, uniform microvilli on the jejunal villi in suckling pigs at 2 and 21 d, with markedly reduced lengths upon weaning. Jejunal villi were shorter in weaned compared with suckling pigs at the same approximate chronological age. Scanning electron microscopy in suckling pigs at 2 and 10 d of age demonstrated long, thin, fingerlike villi with subsequently reduced heights and larger diameters by 35 d of age. At weaning, villi were in close apposition, resulting in an overall smoother villus luminal surface. Villus height subsequently increased by 14 d postweaning, coinciding with the appearance of morphologically tongue-shaped villi. Starter diet composition initially did not influence the villus height reduction response postweaning. Dietary corn oil addition was subsequently associated with shortened villus length (P less than .05) during the starter phase.