Heavy physical exercise may cause gastrointestinal signs and symptoms, and, although splanchnic blood flow may decrease through redistribution by more than 50%, it is unclear whether these signs and symptoms relate to gastrointestinal ischemia. In 10 healthy volunteers, we studied the effect of exercise on gastric mucosal perfusion adequacy using air tonometry. Two relatively short (10 min) exercise stages were conducted on a cycle ergometer, aiming for 80 and 100% of maximum heart rate, respectively. The intragastric-arterial PCO₂ gradient (PCO₂) was elevated by 1.1 ± 1.0 kPa over baseline values (0.1 ± 0.3 kPa) only after maximal exercise (P < 0.001). PCO₂ positively correlated with the arterial lactate level taken as an index of exercise intensity (Spearman's rank test: r = 0.76, P < 0.0001). By bilinear regression analysis, a lactate level of 12 mmol/l, above which a sharp rise in the PCO₂ occurred, was calculated. We conclude that, in healthy volunteers with normal splanchnic vasculature, gastric ischemia may develop during maximal exercise as judged from intragastric PCO₂ tonometry.