An intravenous tryptophan tolerance test (ITrpTT) was designed for liver function since 95% Trp is metabolized by the liver. After an intravenous loading dose of 4 mg/kg body weight, serum levels of both free (F) and total (T) Trp were determined at 45 and 60 minutes. In normal controls and nonhepatic-disease patients, F45 (60) and T45 (60) did not exceed 7 mumol/L and 80 mumol/L, respectively, and F/T ratio not greater than 0.14. These were set up as cutoffs of upper normal limits. The test was abnormal in 87.5% of chronic persistent hepatitis (CPH) characterized by elevation of T45 (60), 100% of chronic active hepatitis (CAH) by elevation of F45 (60) and/or T45 (60), and 100% of hepatic cirrhosis by increase in F45 (60) and F/T ratio but not in T45 (60). The test seems to be more sensitive than the conventional tests for liver function. However, one should be cautious in interpretation of the test as there are some factors which might influence the Trp metabolism like exercise, alcoholism, corticosteroids and enzyme-inducers. It merits as an indication of liver dysfunction only when these factors are considered and excluded.