Because it has been suggested that an accumulation of false neurochemical transmitters plays a part in the cardiovascular and neurological complications of cirrhosis it appeared worthwhile to study the level of monoamine oxidase in the liver and platelets of cirrhotic patients. In fact a decrease of such activities might explain the increase of false neurotransmitters observed in cirrhotic patients. Other enzyme were also tested: the plasma benzylamine oxidase activity and the liver mixed function oxidases. 13 cases of severe cirrhosis (B, C according to Child classification) and 10 cases of less severe cirrhosis (A according to Child classification) were studied in comparison to patients affected by cholelithiasis. A defect in the level of monoamine oxidase activity of platelets and liver was observed in some cirrhotic patients together with a decrease of the level of the liver mixed function oxidases.