Objective: Atrial natriuretic peptide (ANP) via its guanylyl cyclase-A (GC-A) receptor and intracellular cyclic GMP formation participates in regulation of blood pressure and vascular volume. Our previous studies demonstrated that concerted renal diuretic/natriuretic and endothelial permeability effects of ANP cooperate in intravascular volume regulation. Here, we investigated whether ANP stimulates the paracellular or transcellular, caveolae-mediated endothelial transport of albumin and which postreceptor signalling pathways are involved. Methods: We studied the effect of ANP on microvascular permeability to fluorescein isothiocyanate (FITC)-labeled albumin using intravital microscopy on mouse cremaster muscle. These in vivo studies were complemented by studies with cultured microvascular endothelia. Results: Local superfusion of ANP (100 nM) mildly but significantly increased the extravasation of FITC-albumin from postcapillary venules of the cremaster muscle. This response was totally abolished in mice with endothelial disruption of either GC-A or cGMP-dependent protein kinase I (cGKI) or global ablation of Caveolin-1 (cav-1 ^-/- mice), the signature protein of endothelial caveolae. Western blot analyses using isolated caveolae-enriched membrane fractions demonstrated endothelial colocalization of GC-A, cGKI and Caveolin-1. Confocal microscopy showed that ANP increases the uptake of Alexa 488-labeled albumin by primary cultured microvascular lung endothelial cells (MLEC). This effect is abolished in GC-A- or Cav-1 – deficient MLEC or after pretreatment with the Src inhibitor PP2. Conclusion: ANP, via GC-A and cGMP-dependent activation of cGKI, stimulates the caveolae-mediated endothelial transcytosis of albumin. This shifts the balance of hydrostatic and colloid osmotic forces across capillary walls in favour of moving protein-free fluid from plasma into interstitial pools and ultimately decreases intravascular volume. Supported by the Sonderforschungsbereich 688.