SUMMARY We have investigated the effect of inhibition of cellular swelling on subsequent renal function in the presence of marked loss of tissue potassium during 24 hr of simple cold storage of dog kidneys. This was done by initially perfusing kidneys with a solution which contains 130 HIM sucrose and 5 m potassium. Postoperative renal function was equal to that achieved with the “intracellular” solution, C3, which contains 140 DIM glucose and 115 m potassium. Analysis of tissue electrolytes and water showed that the cortex of kidneys stored in the sucrose solution lost the same amount of potassium as the cortex of kidneys stored in lactated Ringer's solution. But while the cortex of kidneys stored in lactated Ringer's solution was swollen by 30%, sucrose cortex, like C3 cortex, lost weight. We also compared the osmotic effect of glucose and sucrose on the water content of anoxic slices of renal cortex at 4 C. Sucrose was the more effective osmotic agent. Finally, we studied the effect of high concentrations of MgSO4 and MgCl2 on changes in cortical potassium and water. Magnesium did not prevent the loss of potassium. But magnesium as well as sulfate did inhibit cellular swelling. We conclude that, during storage, cellular swelling, and not potassium loss or sodium gain, is the primary cause of poor renal function after transplantation. Because “intracellular” C3 solution contains in high concentration several nonpermeable solutes such as gluocse, sulfate, and magnesium, its main beneficial effect during renal storage is not maintenance of cellular potassium, but is, instead, inhibition of cellular swelling.