The symposium will focus on the use of genetically modified mice to investigate the control of vascular permeability and microcirculatory exchange by applying appropriate methods for phenotyping and physiological studies. Recent developments allow multiple, non-invasive measurements of blood to tissue exchanges using advanced techniques based on magnetic resonance imaging, micro-positron emission tomography, and long wavelength fluorescence imaging. The symposium will begin with an overview of new approaches enabling investigations of common mechanisms controlling vascular permeability at the molecular and cellular level, in individually perfused microvessels, and in whole organs. The symposium will then focus on more detailed presentations of recent investigation of several different models. First, genetically modified mice and new imaging techniques are now opening new and unexplored avenues for studying the in vivo function of the role of the family of natriuretic peptides which control the plasma volume by modifying vasodilation, vascular permeability, and renal excretion of water.