Non-toxic cultured juvenile kusafugu Fugu niphobles were fed with a diet containing highly toxic natural komonfugu Fugu poecilonotus liver until the 30th day (8.0 μg of TTX and 3.7 μg of 4,9-anhydroTTX/fish/day), and then fed with a non-toxic diet until the 240th day. During the 30–240th day, five or six fish were periodically sampled six times, and the contents of TTX and 4,9-anhydroTTX in each tissue were determined. The total TTX and 4,9-anhydroTTX accumulated in all tissues tested was not significantly changed during the experimental period, both being kept at 70% of administrated doses. However, in the liver, the TTX content accounted to be 120 μg (50% of administrated) on the 30th day, and then it gradually decreased to 50 μg until the 240th day, while 4,9-anhydroTTX content was kept at approximately 40 μg (40% of administrated) during all the experimental periods. In contrast to the liver, in the skin, TTX and 4,9-anhydroTTX were 40 and 5 μg, respectively, on the 30th day, and then gradually increased to 80 and 24 μg, respectively, until the 240th day. In the intestine, TTX and 4,9-anhydroTTX contents were kept at 25 and 12 μg, respectively, during all the experimental periods. According to these results, we assumed that a part of TTX accumulated in the liver was slowly transferred to the skin.