Using 31P nuclear magnetic resonance spectroscopy, we compared the state of the high-energy phosphates in rabbit kidneys stored at 4 degrees C for 24 hr with 3 different solutions: Ringer (Rg), University of Wisconsin (UW), and Euro-Collins (EC) solutions. We found the highest phosphomonoester/inorganic phosphate (MP:Pi) ratio in the group of kidneys stored in the Rg solution (Rg, 0.93 +/- 0.04; UW, 0.36 +/- 0.02; EC, 0.28 +/- 0.02). This medium has been demonstrated in previous physiological studies to give poor results in terms of organ preservation compared to the solutions that mimic the "intracellular" fluid, such as the EC and UW solutions. Because the commonly used cold storage solutions contain phosphates, which superimpose on the intracellular Pi and, thus, can distort the results, we attempted to eliminate the contaminating solution around the kidney and in the vasculature by flushing the kidney with a phosphate-free solution (Rg). The MP:Pi ratio increased in the UW and EC groups (UW, 0.82 +/- 0.04; EC, 0.64 +/- 0.04) in identical proportion in the 2 groups. It remained highest in the Rg group (1.02 +/- 0.03). Comparisons of data before and after flush showed that external phosphate contamination was not predominant. There was no equilibrium in phosphate distribution between intra- and extracellular spaces at 24 hr of storage. We conclude that the validity of the MP:Pi ratio, as a viability index of renal transplant, might have to be restricted to comparisons of kidneys preserved in the same storage conditions. Therefore, it would be necessary to establish normal and pathological values of this ratio for each cold storage solution.