Different N sources (NO3^-, NH4⁺, or NH4NO3) at relative addition rate (RAR) of 0.25 day^-1 were supplied to tomato (Lycopersicom esculentum Mill.), a species sensitive to NH4⁺ toxicity. At RAR 0.25 day^-1 growth reduction occurred by NH4⁺ application, slightly. Tomato roots had a more efficient NH4⁺ assimilation than shoots when N was supplied solely in the form of NH4⁺, and dominant amino acids in tomato were glutamine and asparagine. Low tissue levels of calcium and magnesium in the NH4⁺-fed plants constituted part of the NH4⁺-toxicity syndrome. The supply of NO3^- led to the accumulation of Fe in tomato roots, but leaves Fe concentration in NO3^--fed plants was low in comparison to NH4⁺ supply. It is concluded that due to the high sensitivity of tomato to NH4⁺, even at RAR 0.25 day^-1 the relative addition rate technique was not able to reduce the deleterious effects of NH4⁺ in tomato plants.