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I nflammatory bowel disease (IBD) is associated with mucosal T cell activation and diarrhea. We found that T cell activation with anti-CD3 mAb induces profound diarrhea in mice. Diarrhea was quantified by intestinal weight-to-length (wt/l) ratios, mucosal Na⁺/K⁺-ATPase activity was determined and ion transport changes were measured in Ussing chambers. Anti-CD3 mAb increased jejunal wt/l ratios by more than 50% at 3 hours, returning to base line after 6 hours. Fluid accumulation was significantly reduced in TNF receptor-1 (TNFR-1^–/–), but not IFN-γ knockout mice. Anti-CD3 mAb decreased mucosal Na⁺/K⁺-ATPase activity, which was blocked by anti-TNF mAb and occurred to a lesser degree in TNFR-1^–/– mice. Neither α nor β subunits of Na⁺/K⁺-ATPase decreased in abundance at 3 hours. Intestinal tissue from anti-CD3–treated mice exhibited increased permeability to mannitol at 1 hour and decreases in electroneutral Na⁺ absorption, Na⁺-dependent glucose absorption, and cAMP-stimulated anion secretion at 3 hours. Furthermore, enteral fluid accumulation was observed in CFTR^–/– mice, indicating a minor role of active anion secretion. These data suggest that diarrhea in IBD is due to TNF-mediated malabsorption rather than to secretory processes. T cell activation induces luminal fluid accumulation by increasing mucosal permeability and reducing epithelial Na⁺/K⁺-ATPase activity leading to decreased intestinal Na⁺ and water absorption.