Fluoride (F–) deposits formed, e.g., in bones, have hitherto mainly been analyzed in highly mineralized tissues obtained from mammals and only rarely from birds. Under certain conditions, F– may also accumulate significantly in the brain and pineal gland and interfere with the secretion of melatonin by that gland. The mammalian and avian pineal gland is called the fifth mineralizing tissue, since it is one of the tissues in the body in which calcification occurs physiologically. In the pineal gland of the elderly, remarkably elevated F–levels occur, reaching as high as 875 mg/kg gland weight and in the pineal calcifications >20,000 mg/kg. However, fluorides have not yet been measured in brain and pineal gland of wild ducks. The aim of our study was to assay and compare F– concentrations in the brain and the pineal gland in adult ducks of two species, one wintering in the Odra estuary (tufted duck, Aythya fuligula, n = 44) and the other in the Pomeranian Bay (velvet scoter, Melanitta fusca, n = 9) of northwestern Poland. The material was collected during the years 2008-2010. F– was determined by a potentiometric method (using the Orion electrode) and expressed as concentration in dry weight, with the assumption that the duck brain contains 79% H2O. In the tufted duck, medians of F– concentration in the brain and pineal gland were 203 mg/kg (range 77–593 mg/kg) and 2,360 mg/kg (range: 674–38,645 mg/kg), respectively. In the brain and pineal gland of the velvet scoter, medians of 174 mg F–/kg (range: 93–551 mg/kg) and 462 mg F–/kg (range: 176–2,269 mg/kg) were found, respectively. In contrast to F– levels in the pineal gland, F– levels in the brain appeared to show significant differences between the two duck species.