Near-infrared (NIR) spectroscopy was applied to rat liver allografts for assessing nitric oxide (NO) synthesis and tissue oxygenation as a means of monitoring the rejection response following liver transplantation. Orthotopic liver transplantation was performed in rats, which were assigned to three groups as follows: group 1, a syngeneic combination (lewis to Lewis); group 2, an allogeneic combination (ACI to Lewis); and group 3, an allogeneic combination treated with 15-deoxyspergualin. NIR spectroscopy was performed on the grafts in recipients, and the relative changes in nitrosyl-Hb (NO bound to erythrocyte hemoglobin), oxy-Hb, and oxidized cytochrome oxidase (Cyt.aa3) were obtained. The level of nitrosyl-Hb was significantly elevated from postoperative day (POD) 3 in group 2 compared with that in group 1, which remained constant (P < 0.05). In group 3, the elevation was significantly suppressed. These data indicate that the alloimmune response is associated with a dramatic change in NO synthesis in grafted livers. In a separate experiment, NO synthesis was also increased after long cold preservation (24 hr) in syngeneic liver transplants. However, the increase was transient and subsided on POD 3. Levels of oxy-Hb and oxidized Cyt.aa3 in group 2 were significantly decreased when parenchymal disorder was confirmed histologically (POD 6 and 8), compared with those in group 1, which remained constant (P < 0.05). In group 3, both of these levels showed improvement. Thus, our NIR spectroscopy technique was shown to be capable of assessing simultaneously both the immune response and the degree of immune-induced destruction of allograft tissue following liver transplantation through monitoring of NO synthesis and tissue oxygenation.