Patients with active inflammatory bowel disease are often reported to be in negative nitrogen balance. Therefore, we examined basal and amino acid stimulated urea synthesis in 11 patients with active inflammatory bowel disease and in 10 patients with non-active disease. A primed continuous infusion of an amino acid mixture was given from t=1 h to t=5 h; during the first and the last two hours no amino acid infusion was given. Urea nitrogen synthesis rate was quantified independently of changes in blood amino acid concentration by means of the functional hepatic nitrogen clearance, i.e. the linear slope of the regression of urea nitrogen synthesis rate on blood amino acid concentration. Basal and amino acid stimulated urea nitrogen synthesis rate as well as functional hepatic nitrogen clearance were elevated two-fold in the patients with active disease. No differences between the two groups were observed as regards basal or stimulated plasma glucagon, cortisol, catecholamines and serum levels of interleukin-1α, interleukin-1β, tumor necrosis factor-alpha and interleukin-6. The results show that liver function related to conversion of.