The experiments of short-term exposure of Mytilus trossulus to cadmium in sea water were carried out in laboratory conditions. The mussels were picked up from the Gulf of Gda?sk and kept in the seawater of temperature 10oC and salinity 7%. The solution of CdCl2 containing radioactiwe isotope of cadmium 115mCd (T1/2 = 43 days) was added to the water and used as a metal tracer. The isotope's specific activity was in the range from 2000 to 10000 Bq115mCd?dm-3, which is equal to the range of 2-10 mgCd?dm-3 of stable cadmium in the water. The time of exposure varied from 10 minutes to 24 hours. A distinct increase in the specific activity of radiocadmium in the soft tissue of the analysed mussels was observed. Thin increase was proportional to the increase in the specific activity of cadmium in water and to the time of exposure. The distribution of the accumulated metal in various inner organs of mussels was not stable in time. At the beginning of exposure (first hour) the biggest concentration of the metal was noticed in gills, smaller in hepatopancreas and the smallest in the remaining soft tissue containing mostly mantle and muscles. After the sec-ond hour and during the remaining 23 hours of exposure the biggest amounts of the accumu-lated metal were found in hepatopancreas. The concentration of radiocadmium in the rest of the soft tissue remained the lowest throughout the exposure.