The isolated rat kidney perfused at 37 C with dialyzed bovine serum albumin (6.5 g/100 ml) in Krebs-Henseleit buffer was used to examine why a hypertonic citrate flush permits rapid recovery of renal function after storage. The composition of the original hyperosmolar citrate solution was varied so that the roles of osmolality, magnesium, and citrate could be evaluated. All kidneys were flushed with the test solutions and stored for 24 hr in the test solutions at 0 C. The citrate flushing solution requires both the citrate anion and magnesium for efficacy. Hyperosmolality does not enhance its action, an isosmolar solution is more effective. Citrate can be replaced by a nonmetabolizable analogue, tricarballylate, if the solution is suitably buffered. The mechanism of action of citrate is still uncertain, it does not seem primarily to act as a metabolic fuel or inhibitor.