Sedentary behaviour has deleterious effects on the cardiovascular system, including reduced endothelial functions. A 2-month bed rest study in healthy women (Wise-2005 program) presented a unique opportunity to analyze the specific effects of prolonged inactivity without other vascular risk factors on the endothelium. We investigated endothelial properties before and after 56 days of bed rest in 8 subjects who performed no exercise (control group: No-EX) and in 8 subjects, who performed regularly treadmill exercise in a Lower Body Negative Pressure chamber, as well as resistance exercise (countermeasure group: EX). A functional evaluation of the microcirculation in the skin was assessed with laser Doppler. We studied endothelium-dependent and endothelium-independent vasodilation using iontophoresis of acetylcholine and sodium nitroprusside, respectively. We also measured circulating endothelial cells (CECs), an index of endothelial damage. In the No-EX group, endothelium-dependent vasodilation was significantly reduced (+35.4 +/- 4.8% vs. +24.1 +/- 3.8%, P < 0.05) by bed rest with a significant increase in the number of CECs (3.6 +/- 1.4/ml vs. 10.6 +/- 2.7/ml, P < 0.05). In the EX group, endothelium-dependent vasodilation and number of CECs were preserved. Our study shows in humans that prolonged bed rest causes impairment of endothelium-dependent function at the microcirculatory level, along with an increase in circulating endothelial cells. Microcirculatory endothelial dysfunction might participate in cardiovascular deconditioning, as well as in several bed rest-induced pathologies. We therefore conclude that the endothelium should be a target for countermeasures during periods of prolonged deconditioning.