New methods to measure vascular permeability in mice enable longitudinal investigation of changes in permeability in the same animal, and distinguish tissue uptake, due to increased microvessel permeability from increased surface area for exchange. One method measures fluorescent intensity (FI) in skin after macromolecules labeled with long-wavelength fluorescent tracers are rapidly injected iv. Permeability is quantified within sub-regions of images of the fluorescent tracer distribution using the initial step increase in FI (measures local intravascular volume) and the subsequent increase of FI (due to blood to tissue efflux of tracer). If increased, the ratio of efflux to local vascular volume is a better measure of real changes in permeability than tissue uptake alone (e.g. using the Miles Assay: see Bates, Cardiovascular Research, 2010). With narrow-band filters, solutes labeled with different fluorescent tracers are injected sequentially for longitudinal measurement of permeability, or used in combination with cells expressing fluorescent protein markers (Kim et al, Am. J. Physiol., 2009).The approach has been extended for internal organs using macromolecules with tracers appropriate for MRI (Curry et al, J. Physiol, 2010; Cardiovascular Research,2010) or micro-PET (Rygh et al, Clinical Cancer Research, 2011) and tested using two-tracer standards (Lin et al, J.Physiol, 2011). Collaborations using these methods facilitate investigations of normal and dysfunctional endothelial barriers in multiple mouse organs. HL28607