A biochemical assay of renal viability previously described for rabbits is validated in a canine model using anoxia, hypothermia, and hypothermic perfusion. The assay correlated well with the decree of anoxic injury and protective effect of hypothermia. It was found to be unable to document significant injury when pulsatile perfusion was added to the system and resulted in nonviable canine kidneys. Apparent vascular damage with interstitial hemorrhage occurred after the perfusion and was not predictable by the use of the assay system. The limitations of any assay for organ viability after complex preservation maneuvers is the result of a multiplicity of injurious factors surrounding the preservation or as a consequence of the preservation. No single assay system will be adequate for protection of organ viability during or after preservation as long as the types of injury are multiple. Is is suggested that great care be used in defining possible injurious factors associated with given preservation maneuver and that specific assays be utilized to document the effects of each of these factors. The more complex the preservation system employed; the more complex the assay system needed.