Direct microscopy will be used to define both the functional and morphometric changes in the microcirculation of normotensive as opposed to hypertensive rats. Simultaneous recording of pressure, flow and vessel dimensions will be made for all blood vessels beginning with 100 mu m arterioles through the capillaries and into the large venules (100 mu m). From pressure and flow determinations "resistance" values will be calculated for the successive branchings of the small vessel network. With this background, specific stimuli will be introduced to further verify the extent and site of pressure-flow derrangements. Clues for the chronological sequence of events in the evaluation of hypertension will be sought by examining the microvasculature in skeletal muscle of 5 week, 10 week and 15-18 week old animals.