We propose to study the mechanisms that control capillary blood flow and transcapillary exchange in mammalian skeletal muscles and other organs and tissues. Our goal is to relate blood-tissue exchange kinetics to the organization of the microvascular network and to the structure of the tissue or organ. Direct measurements of uptake or washout of diffusible substances will be made. In experiments terminated at different durations, histological techniques will be used to study distribution of non-diffusible and diffusible tracers within the tissue. By comparing patterns of distribution under different experimental conditions, we hope to obtain new information about the origin and significance of observed non-uniformities of microvascular flow and transport.