The importance of the cation content of flush solutions used for simple hypothermic storage has been investigated in a 48 hour canine kidney preservation model. Uniformly successful 48 hour storage was achieved with solution C2. However, inclusion of procaine (0.1 Gm. per 100 ml.) in the flushing solution as in C3 produced significant kidney damage. Without procaine, Sacks solution was no better than was C2 and the latter proved significantly superior to a sodium-based solution having identical anion content and osmolarity. The magnesium content of the sodium appeared to favorably influence the outcome of the kidney storage experiments.