Caveolae are -shaped vesicular structures postulated to play a role in transvascular protein transport. Studies on mice lacking endothelial caveolae, caveolin-1 knock out mice (cav-1-KO), indicate increased macromolecular transport rates. This was postulated to be due to the appearance of an alternative pathway. The present study tested if an alternative pathway had appeared in cav-1-KO. Male cav-1-KO (n=12) and male control mice (n=13) were intubated and anesthetized using 2% isofluran. ¹²⁵I-albumin, ¹³¹I-immunoglobulin M (IgM) and polydisperse FITC-Ficoll were administered i.v. During tracer administration, a 90 min peritoneal dialysis dwell was performed. Clearance of tracers to dialysate, and permeability-surface area product (PS) for glucose were assessed. Transvascular protein transport was higher in cav-1-KO compared to control. Albumin clearance from plasma to peritoneum was 0.088±0.008 µl/min in control, and 0.179±0.012 µl/min in cav-1-KO (p=0.001). IgM clearance was 0.049±0.003 µl/min and 0.083±0.010 µl/min in control and cav-1-KO, respectively (p=0.016). Ficoll clearance was increased in cav-1-KO mice. In conclusion, the lack of caveolae in cav-1-KO resulted in a marked increase in macromolecular transport. A two-pore analysis of the Ficoll clearance data revealed that the higher transport rate in cav-1-KO was not compatible with the appearance of an alternative pathway for macromolecular transport. In contrast, the higher transperitoneal protein and Ficoll clearance is consistent with passive porous transport through an unperturbed two-pore system, presumably at an elevated capillary hydraulic pressure. Alternatively, the data may be explained by reductions in the selectivity of the endothelial glycocalyx, leading to an increased capillary hydraulic conductivity and large solute filtration.