The wall effective permeability of hydrophilic substances labeled with fluorescent dyes was evaluated in an isolated cannulated rat single lymph vessel through a videomicroscope system. Sodium fluorescein (NaFl; 332 mol wt) and FITC-dextrans (4,400, 12,000, and 71,200 mol wt) were administered into the intraluminal space of the lymph vessels and then excited by a Xenon lamp. Changes in the fluorescence intensity of the dyes were continuously measured by a silicon-intensified target camera through appropriate filters. The net flux of each dye in the wall of the lymph vessels was calculated by the relationship between the fluorescence intensity and the concentration of the dyes. NaFl and FITC-dextran 4,400 in the intraluminal space of isolated rat lymph vessels significantly penetrated the wall of the lymph vessels. FITC-dextran 12,000 in the intraluminal space of isolated rat lymph vessels slightly passed through the lymphatic wall, whereas FITC-dextran 71,200 did not penetrate the wall. Intraluminal pressures ranging from 4 to 8 cm H(2)O did not significantly affect the net flux of dyes used in the present study. After administration of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate into the lymph vessels, the net flux of FITC-dextran 4,400 and 12,000 but not 71,200 was augmented significantly. These results suggest that small molecular hydrophilic substances (< or =4,400) are permeable from the intraluminal to extraluminal space of isolated lymph vessels and that the endothelial cell surface structure may play a barrier role in the effective permeability of large molecular hydrophilic substances (4,400 to 12,000) through the wall of the lymph vessels.