The possibility of in vivo conversion of inorganic mercury to MeHg in brook trout (Salvelinus fontinalis) was tested by chronically exposing fish to mercury (from Hg(NO3)2) at a concentration of 1 ng/ml, for 56 days, and then maintaining them at background levels for 294 days. The concentrations of inorganic and methylated mercury (MeHg) in several organs of control and test fish were periodically measured over this 50-wk period. The inorganic mercury levels in test fish increased rapidly during exposure. The MeHg concentrations in both control and test fish increased slowly and at the same rate, with final 350-day concentrations comparable to levels found in wild brook trout from an uncontaminated area. This information indicates that fish do not methylate inorganic mercury in vivo, and that approximately one-half of the MeHg body burdens in the fish was acquired via the food.