The role of recipient hemodilution for postischemic renal medullary red cell trapping was investigated after different periods of cold storage in a conventional cold storage solution (Sacks'). At all cold storage times investigated (4, 12, 24, and 48 hr) medullary red cell trapping was reduced by isovolemic hemodilution, with about 50% reduction of recipient hematocrit. Trapping was also reduced when a modification of a new preservation solution (University of Wisconsin solution [UW]) was used and compared with flush-out and storage in a standard preservation solution (Sacks'). The combination of hemodilution and preservation in modified UW solution had additional capacity to reduce medullary red cell trapping. Thus, even after 48 hr of cold storage, only a moderate trapping was observed. The results also indicate that measurements of medullary red cell trapping offers an accurate method of grading postischemic renal damage.