The liver tissue slices of pufferfish accumulate in vitro tetrodotoxin (TTX), when incubated with minimum essential medium containing TTX. In the case of Takifugu rubripes liver slices incubated at a concentration of 25 microg TTT/ml, TTX of 3.9 microg/g was first detected at 2h and increased to 15 microg/g at 48h. The TTX content accumulated was not decreased, even when the slices were further incubated without TTX for additional 48h. Another species of pufferfish T. paradalis also showed similar trend in TTX accumulation, except they accumulated higher concentration of TTX (36.4 microg/g at 48h) than T. rubripes. On the contrary, in the cases of the liver slices from parrot-bass Oplegnathus fasciatus, green ling Hexagrammos otakii and filefish Thamnaconus modestus incubated at a concentration of 25 microgTTX/ml, TTX of 3-4 microg/g was detected even at 0.5h. However, no significant change in TTX contents was recognized during the incubation for 48h. Further incubation of the filefish liver slices without TTX for additional 48h did not decrease the TTX content. It is unlikely that the liver slices of filefish as well as pufferfish rapidly excrete TTX. These results suggest that the difference in the accumulation of TTX between pufferfish and filefish livers is ascribable to the difference not in the TTX excreting ability but in the ability to take up TTX.