- PURPOSE: To assess the dosimetry of Ra-226 in a two-year chronic ingestion study in laboratory maintained fathead minnow fed environmentally relevant levels of the isotope known to occur in gut contents of fish from a uranium mining area. METHODS: Fish were established on reconstituted commercial fish food containing 10 mBq-10 Bq Ra-226.g(- 1) dry food. The fish were sampled at 1, 6, 18 and 24 months and the Ra-226 levels in the whole fish were directly determined using measurement performed using inorganic mass spectrometry. Pilot experiments using higher doses were also done during development of a liquid scintillation detection system which support some data. RESULTS: The data show that after 1 month the levels of accumulation in the fish were below detection. At 6 months there was an activity dependent accumulation which was relatively higher in the low activity groups. By 18 and 24 months the radium was very low in all groups - well below 6 month levels suggesting considerable loss of radium from the fish. These data were confirmed in a small and shorter study using higher dietary activities. The highest dose calculated for any measurement point was 16 μGy.h(- 1) in the 6-month-old fish fed the diet containing 10 Bq.g(- 1). CONCLUSIONS: We conclude that environmentally relevant levels of Ra-226 have a maximum impact at early time-points when the fish are still growing. After that they appear to depurate accumulated radium. In terms of environmental impact, the maximum accumulation peaks at the age where fish could be spawning but is extremely low leading to μGy.year(- 1) doses even after exposure to the high activity diets.