The influence of operative ischemia time on the survival and function in (Lewis × Brown Norway) F1 (LEW × BN) hybrid rat kidneys was studied in Lewis recipients. Thirteen bilaterally nephrectomized rats were allografted with a hybrid kidney using a modified microvascular technique. Total operative ischemia time was kept below 25 min (short ischemia time group). Fourteen additional animals were allografted with kidneys kept in saline for 45 min at 4 C before grafting (total ischemia time 60–75 min). Animals in the long ischemia time group survived over twice as long as those in the short ischemia group (median survival time 41 days versus 12 days, P < 0.001). In the short ischemia time group, the blood urea nitrogen rose progressively after transplantation. In the long ischemia time group, it rose until the 7th to 11th days after transplantation, then fell to levels 2 to 3 times normal, then rose again progressively until the death of the animals. All allografted kidneys showed histological signs of acute rejection; the long ischemia time group showed additional changes of chronic rejection with hyalinization of glomeruli and fibrosis of small arterioles. Animals in both groups showed a rise in anti-BN hemagglutinating and cytotoxic antibodies, but there was no correlation between antibody titer and graft survival. Prolongation of operative ischemia time to a degree which does not directly impair graft function may nevertheless alter graft immunogenicity by effects on passenger leukocytes, or may foster release of surface antigens leading to immunological self-enhancement.