Sodium fluorescein was used as a fluorescent tracer to study the permeability of blood-brain barrier (BBB) and blood spinal cord barrier (bscb) in rodent models. Using the dye as probe substrate, the hepatocyte drug transport of mediated by organic anion transport polypeptide (OATP) was studied. Sodium fluorescein is also used to restore the in situ solute into the bone through fluorescence after photofading. Fluorescein sodium salt is used as a solute to study microvascular permeability. It has been used as a test molecule to study the permeability of blood barrier.