[en] It is of importance to do the basic investigation on the cycling of radioactive nuclides in terrestrial environments. Up to this time, accumulation, excretion and distribution of radioactive nuclides such as ¹³¹I, sup(115m)Cd, ⁸⁵Sr, ²⁰³Hg, ¹³¹Ba, ²²Na and ⁵⁸Co in fresh water fish have been investigated. This paper concentrates in accumulation and excretion of ¹³¹I, and distribution and excretion of ²⁰³Hg-labelled methyl mercury chloride in fresh water fish. It was observed that, under co-existence of IO₃^-, the concentration factor of ¹³¹I concerning fresh water fish came near to that of marine fish as if the accumulation of ¹³¹I were stimulated by IO₃^- ions. Clear difference was recognized on the excretion patterns of ¹³¹I between fresh water fish and marine fish. In case of fresh water fish, ¹³¹I was excreted much more through kidney than through gills. Biological half-life of ¹³¹I from whole body of fresh water fish was about 15 days. Transfer of ²⁰³Hg labelled methyl mercury chloride to the brain of carp was investigated by means of abdominal administration of the nuclide. Not so much accumulation was obtained in the brain. Biological half-life of the nuclide from the brain was calculated as 198 days. (author)