AIM: Aquatic exercise might improve endothelial function due to hydrostatic pressure increasing blood flow and causing shear stress to the endothelium. However, the acute effect of aerobic exercise in water on endothelial function is unclear. The present study compares the acute effect of aerobic exercise at moderate intensity in water and on land on endothelial function. METHODS: Nine healthy young men walked on a treadmill for 30 min while immersed in water up to the xiphoid at 30.0 +/- 0.2 degrees C and on land at an intensity equivalent to 60% heart rate reserve in a crossover trial. Brachial artery flow-mediated dilation (FMD) was measured at baseline and at 30 and 60 min after exercise. Autonomic nervous activity was compared among conditions at the heart rate variability (HRV) during exercise. RESULTS: FMD significantly decreased at 30 min after exercise on land trial (p < 0.05), but did not change after the aquatic trial. However, FMD was significantly higher after aquatic trial than land trial (p < 0.05) at 30 and 60 min after exercise, whereas heart rate, blood pressure, and HRV did not significantly differ between them. CONCLUSION: These findings demonstrate that aerobic exercise in water suppressed the decrease in FMD compared with that on land, regardless of autonomic nervous activity.