Head-down bed rest (HDBR) increases plasma levels of Atrial Natriuretic Peptide (ANP) and decreases norepinephrine levels. ANP promotes lipid mobilization and utilization, an effect independent of sympathetic nervous system activation, when infused into lean healthy men. The purpose of the study was to demonstrate that a physiological increase in ANP contributes to lipid mobilization and oxidation in young men. Eight men where positioned in a HDBR position. Indices of lipid mobilization and hormonal changes were measured in plasma. Extracellular glycerol, an index of lipolysis was determined in subcutaneous adipose tissue (SCAT) using a microdialysis technique. A two-fold increase in plasma ANP concentration was observed after 60 min of HDBR and a plateau was maintained thereafter. Plasma norepinephrine decreased by 30-40% during HDBR, while plasma insulin and glucose levels did not change. The level of plasma NEFA was higher during HDBR. SCAT lipolysis, as reflected by interstitial glycerol, as well as interstitial cGMP, the second messenger of the ANP-pathway, increased during HDBR. This was associated with an increase in blood flow observed throughout the HDBR. Significant changes in the respiratory exchange ratio, and percentage use of lipid and carbohydrate were seen only after 3h of HDBR. Thus, the proportion of lipid oxidized increased by 40% after 3h of HDBR. The rise in plasma ANP during HDBR was associated with increased lipolysis in SCAT and whole-body lipid oxidation. In this physiological setting, independently of increasing catecholamines, our study suggests that ANP contributes to lipid mobilization and oxidation in young men.