Technical Abstract: This study determined quantitative relationships between the plasma urea and amino acid (AA) concentrations and the gut mucosal morphological changes and body weight gains in early weaning (EW) and suckling piglets. The study was conducted for 12 d with 24 Yorkshire piglets of an average initial body weight of 3.4 kg at d 10 of age. The weaned pigs were fed a corn and soybean meal-based diet. There was no sex effects (P>0.05) on any of the endpoint measurements. The EW pig had 32% (P<0.05) of the whole body growth rate of the suckling pigs. The EW pig experienced an elevated whole body AA catabolism (P<0.05) as indicated by significant increases in plasma urea concentration. Weaning decreased (P<0.05) the total counts of mucin-filled goblet cells along the crypt-villus axis. Weaning also caused gut mucosal villus atrophy and crypt hyperplasia (P<0.05) in comparison with suckling piglets. There were linear relationships (P<0.05) between the plasma urea concentration and average daily gain and gut mucosal morphological measurements, supporting the concept that gut mucosal crypt cells are metabolically of a catabolic phenotype, whereas the villus cells are predominantly of an anabolic phenotype. There were also linear relationships (P<0.05) between plasma concentrations of urea and several AA including carnitine, citrulline, glutamate, glutamine, phenylalanine, and taurine. Similarly, there were linear relationships (P<0.05) between the gut mucosal morphological measurements and several AA, with citrulline showing the best correlation. In conclusion, gut mucosal crypt hyperplasia is partially responsible for the weaning-associated elevation in whole body AA catabolism and growth repression. Change in the plasma citrulline concentration is a reliable plasma marker for prediciting alterations of gut mucosal morphology and functions in piglets. Key Words: Citrulline, Gut mucosal morphology, Piglets