Noninvasive techniques have been developed for the assessment of vascular parameters, including vascular permeability, in normal and diseased tissues of mice. In this protocol, mice are injected with an appropriate fluorescent tracer. The effective average vascular permeability (P) of a region of vessels is measured by single-photon microscopy. Additionally, the vascular permeability of individual vessel segments within ~600 µm of a tumor/window interface is measured by multiphoton laser-scanning microscopy (MPLSM).