The effect of FK 506 pretreatment on renal ischemia and reperfusion (I/R) injury was investigated. Adult male rats were assigned to one of two groups (20 animals each). Group 1 (controls) received 0.5 mL saline while group 2 recieved FK 506 (0.3 mg/kg) intravenously 24 h prior to the induction of renal ischemia. After a 60-min period of ischemia of the right kidney, a left nephrectomy was performed. Blood for BUN, creatinine, and tumor necrosis factor (TNF) was obtained prior to ischemia and on days 1, 2, 3, 5, 7, and 10. All surviving animals were sacrificed at day 10. FK 506 pretreatment reduced the serum levels of BUN (p<. 02), creatinine (p<. 02) and TNF (p<. 05) as compared to that seen in controls. Based upon these data, it appears that: (a) renal ischemia induces the release of TNF; (b) FK 506 pretreatment inhibits TNF production; and (c) FK 506 reduces renal injury association with I/R.