[en] A study was carried out to investigate the uptake and release of tritium by the body water and tissue in the fresh water fish under laboratory conditions. Two species of fresh water fishes were used for this experimentation, namely, Helostoma temmincki and Tilapia nilotica. The results obtained were that the uptake of tritium by body water of both fresh water fishes reached equilibrium with the pool specific activity very quickly, i.e. Helostoma temmincki and Tilapia nilotica, both of them showed approximately 4 hours after exposure to HTO to reach equilibrium where as the release of tritium by the body water showed that about 11 hours the activity remained constant after transfer to inactive water. This two species of fresh water fish showed no significant difference in behaviour of uptake and release of tritium by the body water