The effects of additional nitrogen on the toxicity and removal of ferrocyanide and ferricyanide by cyanogenic plants were investigated. Maize (Zea mays L. var. ZN 304) seedlings were grown in the hydroponic solutions with or without additional nitrogen, and amended with either potassium ferrocyanide or potassium ferricyanide at 25.0 +/- 0.5 degrees C for 144 h. Various physiological parameters were monitored to determine the responses of the plant seedlings to the exposure of these two chemicals. A remarkable decrease in transpiration rate, biomass, shoot length, chlorophyll contents, and soluble proteins was evident for maize seedlings grown in the N-free hydroponic solutions spiked with either ferrocyanide or ferricyanide (P < 0.01), but slight changes were observed in the selective parameters in the N-containing hydroponic solutions spiked with either of these chemicals (P > 0.05). A higher removal of ferrocyanide than ferricyanide was registered in the N-free hydroponic solutions, but more ferricyanide than ferrocyanide was removed by maize grown in the N-containing nutrient solutions (P < 0.01). Although roots of maize accumulated iron cyanides, more cyanide was recovered in plant materials of those grown in the N-containing hydroponic solutions than the N-free nutrient solutions (P < 0.05). Mass balance analysis indicated that the majority of the iron cyanides removed from solution was assimilated by maize and additional nitrogen had a significantly negative impact on the uptake of both chemicals (P < 0.01). Results of this study suggest that uptake and assimilation mechanisms for ferrocyanide and ferricyanide might be quite different in maize and the application of the external nitrogen has a substantial influence on the removal of both iron cyanides by plants. None of the iron cyanide complexes can serve as a sole nitrogen source to support maize growth.