Searching for a marker of graft viability, adenosine triphosphate (ATP) content in the graft tissue was measured sequentially during orthotopic liver transplantation (OLT) in 24 adult mongrel dogs. According to the survival period, 12 recipients were divided into two groups: group I - early-death group (n = 6), death within 3 days after OLT and group II - survival group (n = 6), survived more than 7 days after OLT (average 12 days). Serum biochemical findings showed no differences between the two groups except that the prothrombin time on the first postoperative day in group I was significantly longer (p less than 0.05). The ATP content ratio (% of initial value) was significantly higher in group II, compared to that of group I (p less than 0.05), 94 versus 53% at 10 min after perfusion, 31 versus 17% at just before recirculation, respectively. The ATP content ratio recovered significantly to more than 80% at 10 min after recirculation in group II but only to less than 50% in group I. These results suggest that restoration of ATP content in the liver after graft recirculation reflects the viability of the graft and can serve as an appropriate prognostic marker in cases of OLT.