In the Central Amazonian floodplains, several hundred tree species grow in areas that are periodically flooded by nutrient-rich white-water rivers (várzea) and by nutrient poor black-water rivers (igapó). Seed masses of 31 species from várzea and 27 species from igapó were compared taking into consideration their taxonomic relatedness. Overall average seed mass was higher (mean = 7.08 g) in nutrient-poor igapó than in nutrient-rich várzea (mean = 1.16 g). In igapó, the species growing at high elevations on the flooding gradient had significantly higher seed masses than the species growing at low elevations. In várzea, no difference was found between species growing at high and low elevations. Four large-seeded species from igapó occurring at high elevations on the flooding gradient were responsible for most of the difference in average seed mass between forest types. These data suggest that at low positions in the flooding gradient in igapó, selection pressure on seed size is probably the same as in várzea. At sites with short periods of flooding in igapó forests, on high levels in the flooding gradient, the need for rapid height growth may have selected for species with larger seeds which enable seedlings to be less dependent on soil nutrients.