The effect of up to 6 days of hypothermic perfusion and up to 72 h of cold storage on tissue viability were studied from measurements of sodium potassium pump activity and adenosine nucleotide content. For the hypothermic perfusion two different albumin-based perfusates were used, one with, the other without addition of caprylic acid and amino acids. No difference could be found in the parameters studied between perfusion with these two perfusates. For both perfusates total adenine nucleotide content (TAN) decreased by approximately 60% during perfusion while energy charge potential (ECP) and the ATP/ADP ratio remained constant. During cold storage a decrease of both TAN and ECP and the ratio ATP/ADP was seen, the main decrease occurring within the 1st h. The transmembrane potassium influx as deduced from potassium content in incubated cortical slices during steady state conditions decreased during 6 days of hypothermic storage to 24.5 +/- 2.4 mumol/kg which was a significantly higher level than after 24 h of cold storage. Thus, addition of extra substrate to the perfusate does not seem to improve kidney cortex viability during hypothermic perfusion. Parenchymal viability is better preserved with continuous hypothermic perfusion, compared to simple cold storage in a solution of the same ionic composition as that used for perfusion.