The small intestinal mucosal apical hydrolases are essential to the terminal digestion of enteral nutrients such as carbohydrates, proteins, fats and phosphates, and non-immune defense. Weaning results in the complete replacement of fetal enterocytes with mature adult-type enterocytes and is typified by mucosal atrophy, crypt hyperplasia and compromised digestive and defensive functions. Given these severe physiological changes, we hypothesize that the major apical small intestinal hydrolases will be differentially expressed, allowing for reprogramming and adaptation, in the early-weaned piglet. Therefore, the objectives of this study were to examine changes in the digestive capacity, the catalytic kinetics, and abundances of protein and mRNA of the small intestinal hydrolases, including alkaline phosphatase (IAP), lactase phlorizin hydrolase (LPH), sucrase-isomaltase (SI), maltase-glucoamylase (MGA) and aminopeptidase N (APN), in the early-weaned pigs in comparison with suckling pigs. A total of 20 Yorkshire piglets, 10 suckling (SU) and 10 early-weaned (WN) with an average initial body weight of about 3 kg at the age of 10 d, were used in this study. Weanling piglets were fed a corn and soybean meal-based diet for 12 d. Proximal jejunal samples from both groups were collected. Hydrolase kinetic experiments were conducted using the substrates of lactose (0-75 mM), sucrose (0-75 mM), maltose (0-75 mM), amylose (0-100 mM), p-nitrophenyl phosphate (0-10 mM), and L-alanine-p-nitroanilide hydrochloride (0-16 mM). Abundances of the target gene hydrolase protein and mRNA were analyzed by Western blotting and quantitative real time reverse transcription- polymerase chain reaction (RT-PCR), respectively, using ß-actin as a control. Results from this study demonstrate that early weaning down-regulated (P < 0.05) the digestive capacity and expression of LPH while simultaneously increasing (P < 0.05) the digestive capacity and expression of SI and MGA. Furthermore, weaning decreased (P < 0.05) the digestive capacity and expression of APN and IAP by 35 and 50%, respectively. Thus, the early-weaning process differentially affected the expression of the apical membrane-bound hydrolases of the small intestine. The down-regulation of IAP highlights the reduced microbial detoxifying capacity of the newly weaned piglet and provides some insight into the cascade of immune related events that occur during the post-weaning transition period. The reduced expression of LPH and the simultaneous up-regulation of SI, maltase, and MGA indicate the unique nature of the small intestinal reprogramming that occurs during weaning. These results imply that the early weaning events help the small intestine adapt to the transition to starch digestion. Meanwhile, the down-regulation of the APN expression may be partially responsible for the reduced efficiency of whole body protein utilization, and the pervasive localized immune responses observed in the small intestine of early-weaned piglets. / Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Program, and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)-the University of Guelph Partnership Research Program.