We propose to extend our studies of dissipative transport processes in experiments with isolated proximal straight, proximal convoluted and cortical collecting tubule from rabbit kidney. The experiments with the proximal straight and convoluted tubules will involve: a further examination of the role of HCO3 in isotonic volume reabsorption and in the flow-dependence of the passive component of salt absorption; a study of acetate reabsorptic and its role in the flow-dependence of passive salt reabsorption; and, an assessment of the contribution of the active component of salt reabsorption (both in the HCO3 and acetate systems) to the flow-dependence of volume absorption. We anticipate that these studies will enable us to formulate hypotheses or models of the molecular basis of glomerulotubular balance. The experiments with cortical collecting tubules will involve a further examination of the mechanism of water transport, both THO diffusion and osmotic water flow, in the presence and absence of antidiuretic hormone. These studies will include: determination of the activation energy for THO diffusion in the absence of ADH; and the activation energies of a number of moderately lipophilic species (e.g. caffeine and 1,7 heptanediol). We anticipate that these studies will enable us to formulate a detailed model of the ADH-inducible pathway for water permeation across the rabbit cortical collecting tubule.