Serotoninergic mechanisms are thought to play a role in portal hypertension. Because this biomine is metabolized by the liver, peripheral blood and plasma levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid (the main metabolite of 5-hydroxytryptamine) were measured in 30 patients with cirrhosis. Whole-blood 5-hydroxytryptamine levels were significantly lower in patients with cirrhosis (158 +/- 28 nM) than in age-matched controls (332 +/- 19 nM), and no correlation was found between these levels and the severity of cirrhosis. Unconjugated plasma 5-hydroxytryptamine levels, an indication of the active form of 5-hydroxytryptamine, were significantly higher in patients with cirrhosis than in controls (6.8 +/- 1.7 nM and 3.4 +/- 0.5 nM, respectively), and in patients with cirrhosis these levels were higher in Pugh grade A than in Pugh grade C patients. Conjugated-plasma 5-hydroxytryptamine levels were not significantly different between patients with cirrhosis (32.2 +/- 8.1 nmol/L) and controls (16.4 +/- 1.4 nmol/L). Plasma 5-hydroxyindole acetic acid was significantly lower in patients with cirrhosis than in controls (1.5 +/- 0.1 nmol/L and 2.3 +/- 0.1 nmol/L, respectively). In conclusion, this study shows that serotoninergic mechanisms are altered in patients with cirrhosis.