Methionine is a sulfur-containing essential amino acid and there are optic isomers of L- and D-type. It has been known that there are two metabolic pathways in methionine metabolism-the transsulfuration and the transamination. The purposes of the present investigation are to clarify the existence of transaminative pathway in human by the quantitative analysis of 3-methylthiopropionate (3-MTP), one of the metabolites of methionine, and to study its clinical significance in patients with liver cirrhosis. 3-MTP in urine was analysed by gas chromatograph equipped with the flame photometric detector (FPD) which has a highly specific sensitivity to the sulfur compounds. Fasting levels of 3-MTP concentration in urine in healthy subjects (n = 20) and patients with liver cirrhosis (n = 21) were 39.1 +/- 9.7 ng/mg. Cr. (Mean +/- SE) and 103.6 +/- 24.2 ng/mg. Cr., respectively. 3-MTP concentration in urine in cirrhotic patients was significantly higher than that in healthy subjects (p less than 0.05). In some cases, 3-MTP concentration in urine was measured every hour for 3 to 6 hours after the oral loading of 2 g of D- or L-methionine. In healthy subjects, 3-MTP concentration in urine increased remarkably at 1 hour period after oral loading of 2 g of D-methionine, and subsequently its concentration showed a tendency to decrease gradually. However, there were no such changes after oral loading of 2 g of L-methionine. When 2 g of D-methionine was loaded orally, decreasing curves of 3-MTP concentration in urine were different between healthy subjects and patients with liver cirrhosis, and half-disappearance time of 3-MTP concentration in urine was remarkably prolonged in cirrhotic patients. These findings seem to indicate that 3-MTP is one of the metabolites of methionine (especially of D-methionine) and the transaminative pathway exists in human. It was also suggested that there was the impairment of the transaminative pathway of methionine metabolism in patients with liver cirrhosis. Pharmacokinetics of 3-MTP in urine seems to contribute to the clinicopathological investigation of the liver cirrhosis.