The L-arginine/nitric oxide (L-Arg/NO) pathway is altered in liver and kidney diseases. However, the status of the L-Arg/NO pathway during and after orthotopic transplantation is insufficiently investigated and findings are uncertain because of analytical shortcomings. Also, most human studies have focused on individual members of the L-Arg/NO pathway such as nitrate or asymmetric dimethylarginine (ADMA). In the present article we report on a pilot study investigating extensively the status of the L-Arg/NO pathway before and during orthotopic liver transplantation (OLT). By using fully validated, highly sensitive and specific GC-MS and GC-MS/MS methods nitrite, nitrate, ADMA and its hydrolysis product dimethylamine (DMA), L-arginine and L-ornithine were measured in blood and urine. Our study gives strong evidence of the exceptional importance of hepatic dimethylarginine dimethylaminohydrolase (DDAH) activity for the elimination of systemic ADMA. In end-stage liver disease the synthesis of NO and ADMA as well as the DDAH activity are elevated. However, increase in DDAH activity is insufficient to efficiently eliminate overproduced ADMA. The transplanted liver graft is capable of clearing ADMA in a rapid and sufficient manner. In contrast to studies from other groups, our study shows that in OLT as well as in living donor kidney transplantation, the second study reported here, reperfusion of the graft does not cause drastic alterations to the L-Arg/NO pathway with regard to NO synthesis. In the OLT study the concentration of circulating L-arginine fell temporally dramatically, while L-ornithine levels increased diametrically, most likely due to elevation of arginase activity. However, the relatively long-lasting decrease in plasmatic L-arginine in OLT seems not to have affected NO synthesis after reperfusion. Our OLT study suggests that liver reperfusion is associated with greatly elevated activity of proteolytic and hydrolytic enzymes including DDAH and arginase. Suppression of proteolytic and hydrolytic activity in transplantation could be a useful measure to improve outcome and remains to be investigated in further studies on larger patient collectives. The importance of analytical chemistry in this area of research is also discussed in this article.