In this study, we investigated the hepatic uptake clearance (CL(uptake)) of tetrodotoxin (TTX) in the marine puffer fish Takifugu rubripes by integration plot analysis after a single bolus injection of 0.25mg TTX/kg body weight into the hepatic vein at 20 degrees C. The blood concentration of TTX decreased over time after the injection, from 1451+/-45 ng/mL at 10 min to 364+/-59 ng/mL at 60 min. TTX concentrations in the spleen and kidney decreased in parallel with the blood concentrations, whereas those in the muscle and skin remained almost the same throughout the experiment. In contrast, the TTX concentration in the liver gradually increased, reaching 1240+/-90 ng/g liver at 60 min after injection. The amount of TTX that had accumulated in the liver 60 min after injection accounted for 63+/-5% of the administered dose. Integration plot analysis indicated a CL(uptake) of 3.1 mL/min/kg body weight in the liver for TTX, a rate far below that of the hepatic portal vein blood flow rate (at most, 9%). This finding is consistent with negligible extraction of TTX by the liver. The results demonstrated conclusively that the liver-specific distribution of TTX in T. rubripes is achieved by removal from the systemic circulation, but not by the hepatic first-pass effect.