125I uptake by the thyroid was most pronounced in the smelt (Osmerus eperlanus), perch (Perca fluviatilis), rainbow trout (Salmo gairdneri), roach (Rutilus rutilus) and bleak (Alburnus alburnus). Unlike other tissues, only the muscle in the species studied no relative accumulation of 125I as compared to the ambient water. The Crusian carp (Cyprinus carassius) indicated the lowest levels of accumulation of 125I among the species tested. Thyroid radioiodine concentration was independent of the length of the fish in the rainbow trout and perch. In the rainbow trout, however, the liver and muscle radioiodine concentrations decreased significantly with increasing length of the fish. Exposure of rainbow trout to water containing supranormal concentration of nitrate (1500 micrograms/l) resulted in lowered 125I uptake. The same phenomenon was observed for the perch and Crusian carp, with low nitrate concentrations, while with higher nitrate concentrations, the uptake levels were again increased. Thyroid function, as judged from the conversion ratio and T/S ratio, was not affected by nitrate.