Gastrointestinal (GI) permeability refers to the non-mediated diffusion of large molecules (> 150 Da) that are normally restricted from the body's internal environment by the GI barrier. Non-steroidal anti-inflammatory drugs such as aspirin are known to disrupt the GI barrier and increase GI permeability. Increased GI permeability to harmful substances can promote both local and systemic inflammatory reactions. PURPOSE: The purpose of this study was to determine the acute aspirin dose that increases GI permeability. METHODS: Eight subjects (4 male; 4 female; age = 25 ± 7.6 yrs) completed four experiments: 1) 0 mg of aspirin, 2) 325 mg of aspirin, 3) 650 mg of aspirin, and 4) 975 mg of aspirin. In a randomized, blinded study, subjects were given one of the four doses of aspirin 1 hour before bedtime and again the morning of the experiment. On the morning of the experiment, subjects then drank a solution (150 ml) containing 5 g sucrose, 5 g lactulose, and 2 g rhamnose approximately 1 hour after ingesting the pills. All urine was collected for the following 5 hours. Using high-performance liquid chromatography, urine concentrations of lactulose, rhamnose, and sucrose were determined. Gastroduodenal permeability was assessed by urinary sucrose excretion (percent of ingested dose). Small intestinal permeability was determined from the lactulose-to-rhamnose urinary excretion ratio (L/R). RESULTS: Sucrose excretion was found to be significantly greater with aspirin doses of 650 mg (median = 0.24% excreted) and 975 mg (median = 0.36% excreted) compared to the 0 mg aspirin condition (median = 0.007% excreted). In addition, the L/R showed a significant difference between a 975 mg dose of aspirin (median = 0.063) and a 0 mg dose (median = 0.038). CONCLUSION: Gastroduodenal permeability is significantly increased with an aspirin dose of 650 mg (i.e., 2 × 325 mg capsules) when taken before bedtime and the next morning. Small intestinal permeability is also increased with an aspirin dose of 975 mg (i.e., 3 × 325 mg capsules) when ingested in the same manner. Supported by the Gatorade Sports Science Institute.