BACKGROUND Nonsteroidal anti-inflammatory drugs (NSAIDs) are frequently prescribed to dogs for their analgesic, antipyretic, and anti-inflammatory properties. Their beneficial actions can be offset by gastrointestinal (GI) toxicosis. Endoscopy has traditionally been employed to detect GI lesions, but alterations in GI permeability precede the development of mucosal damage. OBJECTIVE Carprofen and meloxicam alter GI permeability and mucosal absorptive capacity of dogs. METHODS Twenty adult dogs treated with an NSAID for >7 days were evaluated by permeability tests while receiving either carprofen (10 dogs) or meloxicam (10 dogs). METHODS Prospective, longitudinal observational study. A 6-sugar permeability test (sucrose, lactulose, rhamnose, 3-O-methyl-D-glucose, D-xylose, and sucralose) was performed on the day before NSAID treatment, and after 3 and 8 days of treatment. RESULTS There were no significant differences in the urinary recovery ratios of lactulose: rhamnose, D-xylose: 3-O-methyl-D-glucose, or sucralose recovery within either group at any time during the study. Sucrose permeability in the meloxicam group did not alter significantly over time. However, sucrose permeability in the carprofen group decreased significantly by day 3 (P = .049) and increased again by day 8 (P = .049), to a level that was not significantly different to permeability before treatment (P = .695). CONCLUSIONS The absence of increased GI permeability and diminished mucosal absorptive capacity in this group of dogs does not support the development of acute GI toxicosis during treatment with either meloxicam or carprofen.