Intra-vital microscopy of the microcirculation in skeletal muscle and mesentery will be carried out in conjunction with direct recording of pressure, flow, diameter and hematocrit under steady state conditions and after the introduction of both acute and chronic perturbations. The general aim is to identify control mechanisms and to characterize their involvement in the distribution of flow, pressure, and, in turn, their effect on transcapillary exchange of fluid. Special attention is to be placed on the development of hypertension in the rat and its association with microvascular resistance with an attempt to distinguish between the changes in the larger arterioles (under neurogenic control) and smaller arterioles (below 30 microns) which are under local autoregulatory influences. Ancillary problems such as the vasomotor response to tissue injury and the pharmacology of microvascular smooth muscle will also be investigated.