Our previous work has demonstrated that weaning at 19 days-of-age has deleterious effects on mucosal barrier function mediated through intestinal CRF receptor signaling pathways in the intestine. The objectives of the present study were to assess the impact of piglet age on weaning-associated intestinal dysfunction and to determine the role that mast cells . Nursing Yorkshire-cross piglets were either weaned at 19 days-of-age (early-weaned, n=8) or 28 days-of-age (late-weaned, n=8) and housed in nursery pens. Twenty-four hours post-weaning, mid-jejunum and ascending colon from piglets within each weaning age group was mounted on Ussing chambers for measurements of transepithelial electrical resistance (TER) and serosal-to-mucosal ³H-mannitol fluxes. Early weaning resulted in reductions in transepithelial electrical resistance (TER) and increases in mucosal permeability to ³H-mannitol in the jejunum and colon (P<0.01). In contrast, post-weaning reductions in intestinal barrier function were not observed in piglets weaned at 28 days-of-age. Early-weaned piglet intestinal mucosa had increased expression of CRF receptor 1 (CRF-r1) protein, increased mucosal mast cell tryptase levels, and evidence of enhanced mast cell degranulation compared with late weaned intestinal mucosa. Pre-treatment of piglets with the mast cell stabilizer drug, cromolyn injected intraperitoneally 30-minutes prior to weaning, abolished the early weaning-induced intestinal barrier disturbances. Early weaning stress induces mucosal dysfunction mediated by intestinal mast cell activation, and can be prevented by delaying weaning.