The isolated rat kidney perfused at 37 C was used to evaluate the effect of adding plasma proteins to, and varying osmolality of, cold-storage flushing solutions with or without buffering. Addition of albumin improved immediate poststorage kidney function (glomerular filtration rate [GFR], fractional sodium reabsorption, and fractional protein clearance) of all flushing solutions tested after 6 hr and 24 hr of storage. At 6 hr, these improvements also correlated with less weight gain. Flushing solutions containing citrate and sulfate produced significantly better return of function after 24 hr of cold storage than Krebs' or Collins'-derived solutions. Osmolality was unimportant with solutions containing citrate. Collins' solution with reduced MgSO4 yielded better poststorage function than conventional solution. An all-citrate isotonic solution buffered with 15 mmol THAM preserved poststorage function at 48 hr better than a similarly buffered solution containing both citrate and sulfate. Loss of dry weight during storage and subsequent perfusion appeared to correlate, in these experiments, with loss of poststorage function. The isolated rat kidney provides discrimination among various flushing solutions. The technique might be useful in the assay of additional variables that might affect the quality of kidney preservation.