Moist-chilled and freshly harvested seeds of redroot pigweed (Amaranthus retroflexus L.) and yellow foxtail [Setaria glauca (L.) Beauv.] were tested for germination responses to soil ammonium and nitrate. A controlled environment was used to simulate overwintering and germination conditions in the field. The results suggest that no alteration of weed germination behavior should be expected in the crop ecosystems studied if ammonium plus nitrification inhibitor replaces other fertilizer systems which result in substantial levels of nitrate in the soil solution. The only germination effect was one of nitrate on unchilled foxtail. Chilling lowered catalase activity in both species.