The overall objective of this project is to analyze renal transport processes by means of a detailed systems model and diverse experimental data. Specifically, the model takes into account medullary architecture and major passive and active membrane transport mechanisms. From model simulations we have found that active transport of urea does not have to exist to explain all experimental observations pertaining to the regulation of urea excretion. To allow efficient computer simulation of transport in the entire renal medulla, we have shown that perfectly mixed compartments in series can be used when the compartment parameters are appropriately related to the tubular membrane transport coefficients, the geometry of the tubules, and the state of the system (i.e., special distributions of concentrations and flows). With this method, model simulations are being carried out to compare hypotheses concerning the requirement of active salt transport from the thin ascending limb. BIBLIOGRAPHIC REFERENCES: Knepper, M.A., G.M. Saidel, and P.J. Palatt (1975). "Systems model of renal regulation of urea excretion." Med. and Biol. Eng. (Accepted for publication.) Knepper, M.A., G.M. Saidel, and R.A. Danielson (1976). "Simulation of tubular mass transport using perfectly mixed compartments". Proc. of the Summer Computer Simulation Conference. Wash., D.C.