Inhibitors of ethylene synthesis and action were used to alleviate ammonium toxicity in tomato (Lycopersicon esculentum Mill. 'Heinz 1350') grown on ammonium-based nutrient solutions. Aminooxyacetic acid and Ag+ were effective in reducing ammonium toxicity, whereas Co+2 and salicylic acid were not. A hypothesis was developed to integrate ammonium accumulation and ethylene biosynthesis into a mechanism for expression of plant injury from environmental stresses.