The toad urinary bladder, a widely employed model for the mammalian kidney, carries out transepithelial active transport of sodium, driven by the metabolism of adenosine triphosphate (ATP) derived from oxidative phosphorylation. Despite intensive study, understanding of the energetics of this process is incomplete, largely owing to the unreliability of methods for the analysis of metabolic intermediates in a functioning tissue. Thus it is not known whether various stimulants and inhibitors modify transport primarily by influencing kinetic (permeability) factors, energetic factors, or both, although each of these mechanisms should have a characteristic effect on the temporal response of metabolite patterns. Preliminary studies in our laboratory have demonstrated the feasibility of using high resolution 31P nuclear magnetic resonance (NMR) to monitor tissue levels of ATP, adenosine diphosphate (ADP), inorganic phosphate (Pi), creatine phosphate (CrP), and sugar phosphates (SP) in a functioning toad urinary bladder over experimentally appropriate intervals of time. Accordingly, it is anticipated that concurrent observations of transport and NMR spectra will characterize the control state and the dynamic response to drugs and hormones, thereby indicating kinetic and energetic factors modulating transport. Furthermore, the comparison of chemical assays of tissue extracts and NMR spectra before and after freeze-clamping the tissues will permit evaluation of the adequacy of standard biochemical procedures for estimating physiological metabolite levels.