Intestinal fatty acid binding protein (IFABP), a 15 kDa protein present in the cytosol of mature enterocytes, is rapidly released into the circulation following enterocyte injury and is a sensitive measure of gastrointestinal (GI) barrier damage following exertional heat stress (1). Exertional heat stress protocols typically employ running or cycling exercise at a set percentage of VO2max, which may introduce systematically different rates of metabolic heat production and differences in core temperature responses between groups differing in biophysical characteristics (e.g. sex, body mass, body surface area) (2). This in turn may affect systemic markers of GI barrier function, which are positively related to core body temperature (3). It has long been assumed that the mechanical stress associated with running provokes greater GI damage when compared to cycling, though there is little data to support or refute this notion. To determine whether exercise mode effects systemic measurements of intestinal damage, four men (height: 180.0 ± 6.1 cm, body mass: 82.5 ± 16.6 kg, cycling VO2max: 46.4 ± 8.4 mL.kg-1.min-1, running VO2max: 53.3 ± 8.5 mL.kg-1.min-1) completed a cycling trial and a running trial at a matched rate of metabolic heat production (9 W.kg-1) in hot, humid conditions (39.2°C, 51.0 % relative humidity). Participants exercised until either a rectal temperature of 40.0°C was obtained, or withdrew due to exhaustion. Blood samples were drawn at rest, when rectal temperature increased by 1.5°C from baseline, and at the end of exercise. IFABP was quantified in serum via enzyme linked immunosorbent assay. Metabolic heat production was similar between running and cycling (running: 9.3 ± 0.4 W.kg-1, 765 ± 137 W, 374 ± 29 W.m2; cycling: 8.7 ± 0.4 W.kg-1, 715 ± 113 W, 354 ± 19 W.m2). Running exercise time was 01:19:39 ± 00:21:20 hh:mm:ss, and cycling exercise time was 01:06:25 ± 00:14:17 hh:mm:ss. Mean exercise heart rate was similar between conditions (running: 146 ± 8 bt.min-1; cycling: 148 ± 8 bt.min-1), as was relative exercise intensity (running: 52 ± 7 %VO2max; cycling: 56 ± 8 %VO2max). Time to 1.5°C was 00:35:52 ± 00:04:31 while running, and 00:35:33 ± 00:08:48 while cycling. Peak rectal temperature (39.39 ± 0.29°C vs 39.00 ± 0.39°C) and delta change in rectal temperature (2.90 ± 0.39°C vs 2.49 ± 0.62°C) were higher in the running vs. cycling trial. IFABP concentration increased by 357 ± 484 pg.ml-1 and 622 ± 642 pg.ml-1 at +1.5°C and exhaustion during cycling, and 97 ± 67 and 378 ± 690 pg.ml-1 at +1.5 °C and exhaustion during running. Despite the longer exercise duration and greater peak core temperature observed during running, the release of IFABP was greater both during and after the cycling exercise when metabolic heat production was matched at 9 W.kg-1.