SUMMARY Formazan production by tissue slices has been proposed as a rapid, prospective method of assessing organ viability prior to transplantation. In these experiments, formazan production by biopsies of canine liver and kidney has been measured by two methods. The biopsies were obtained from the excised organs during storage at 37 C for 60-90 min. Neither method demonstrated a significant change in formazan production during storage under these conditions.