In the microvasculature, endothelial cells are the primary physical barrier between blood and tissues. The other microvessel wall cell is the pericyte. Although much has been learned about the biology of endothelium, appreciably less is known about pericytes. The focus of this study is to demonstrate the: phenotypic expression of specific contractile proteins in pericytes from different microvascular beds; the mechanisms by which pericytes contract and relax; pericyte humoral com- munication with endothelial cells; finally, to correlate these structural, biochemical and physiological characteristics and changes in interendothelial junctional permeability. The experimental protocols involve the culture of pericytes, and when applicable, their coculture with homologous endothelial cells obtained from bovine retina, lung, and adrenal tissues. This sample of tissues reflects microvascular beds with significant degrees of differences in permeability and different pericyte to endothelial ratios. High priority is given to the identity of specific biochemical markers and physiological properties that are related to pericyte contraction, and that may distinguish them from endothelial and vascular smooth muscle cells. Pericyte phenotypes are also thought to be associated with basal inherent microvascular permeabilities as seen in different organs and tissues. Another priority is to demonstrate in the paracrine communication pathways between pericytes and endothelial cells the importance of prostanoids and other agonists that are known to modulate barrier function and to contract or relax one or both cell types. In these experiments, pericytes and en- dothelial cells will be co-cultured to measure cell to cell signaling and pericyte influence on the differentiation of the endothelial junctional cytoskeletal contractile proteins in relationship to macromolecule transport. Pericytes will be cultured on a silicone substrate and silicone deformation will indicate contraction. The second messenger pathway that regulates pericyte contractile protein phosphorylations will be identified. All of the quantitative techniques and culture procedures associated with these goals are routinely performed in the laboratory and have been published.