This invention features in vitro methods of determining the rate and extent to which test compounds, such as drug candidates, enter, move within, and traverse lipid-associated barriers. This invention also features method that measure the accumulation, that is the partition coefficient, of test compounds in lipid structures relative to, e.g., an aqueous phase, as well as the rate of accumulation of test compounds in the lipid structures. In addition, the invention features lipid vesicles that contain fluorophores localized within the aqueous interior that can be employed in the methods described herein.