By means of activation analysis the mercury content in feathers of the following bird species was studied: Phasiamis colchicus, Perdix perdue, Lagopus lagopus, Falco peregrinus, HaliaBtus albicilia and a small material of Emberiza calandra, Asio otus, Strix aluco, Buteo buteo. and Circus cyaneus. Possible sources of post mortem contamination of the specimens are discussed. Allowance being made for such cases, the activation method as applied to the analysis of mercury in feathers appears reliable. Roughly constant Hg levels are encountered in each of the investigated species during the period 1840-1940, although with considerable variation between the species. Differences of position within the food chains seem to be the main cause of the differences in Hg level between species; a well-documented increase of Hg concentration, amounting to at least 10-20 times the previous level, appears in the 1940'' s and 1950'' s, with some time-differences between the species in the available material. The appearance of increased mercury accumulations in birds mainly in the beginning of the 1940'' s indicates that alkyl-Hg compounds used as seed dressings are chiefly responsible for that increase.