Exertional hyperthermia reduces splanchnic blood flow which leads to gastrointestinal ischemia. This is associated with increased gastrointestinal perturbations and endotoxemia which are potential precursors of exertional heat stroke. However, whether hyperthermia alone can result in significant disruption to gastrointestinal integrity remains unclear. PURPOSE: To determine whether exertional or passive hyperthermia contributes to greater changes in gastrointestinal permeability METHODS: 16 physically active males (mean ± SD; age: 26 ± 3, VO2max: 65 ± 6 ml/kg/min) completed three randomised trials. Blood samples were obtained at baseline and after (a) 70% VO2max run (RUN; Tdb: 30.0 ± 0.1 °C, RH: 71 ± 1%), (b) 50% VO2max walk (WALK; Tdb: 30.0 ± 0.0 °C, RH: 71 ± 0%) and (c) passive heating via warm water immersion at 42 °C (PaH). Serum concentrations of I-FABP, LPS and IL-6 were determined using ELISA. Gastrointestinal temperature (Tgi), heart rate (HR), physiological strain index (PSI) and changes in blood biomarkers were assessed using 1-way ANOVA or paired t-test with p < 0.05 as statistically significant. RESULTS: Participants observed similar baseline Tgi (37.0 ± 0.2 °C, p = 0.204) and HR (63 ± 7 bpm; p = 0.118). End Tgi was similar for RUN (39.4 ± 0.2 °C) and PaH (39.3 ± 0.3 °C, p = 0.289), but lower in WALK (37.9 ± 0.3 °C, p < 0.001). Mean PSI was greatest in RUN (7.4 ± 0.5), followed by PaH (5.2 ± 0.6) and WALK (3.1 ± 0.7, p < 0.001). Serum concentrations of I-FABP, LPS and IL-6 were similar at baseline (p > 0.05). Serum I-FABP and IL-6 concentrations increased from baseline to post-trial in RUN (I-FABP: 797 ± 467 pg/ml vs 1801 ± 1432 pg/ml, p = 0.007; IL-6: 4.3 ± 3.4 pg/ml vs 9.3 ± 5.4 pg/ml, p < 0.001) and WALK (I-FABP: 772 ± 460 pg/ml vs 1121 ± 966 pg/ml, p = 0.044; IL-6: 4.5 ± 4.1 pg/ml vs 9.1 ± 8.5 pg/ml, p = 0.004) but not in PaH (I-FABP: 888 ± 572 pg/ml vs 893 ± 550 pg/ml, p = 0.935; IL-6: 5.3 ± 5.4 pg/ml vs 5.5 ± 5.2 pg/ml, p = 0.285). No differences in post-trial serum LPS concentrations were observed (p = 0.337). CONCLUSION: Hyperthermia alone may be insufficient to trigger changes in gastrointestinal permeability. Disruptions to gastrointestinal integrity during exertional hyperthermia may instead be exercise-induced, stemming from mechanical and metabolic stress arising from physical exertion. Supported by NUS Internal Research Funding