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Summary: Nitrogen availability is an important limiting factor for plant growth. Although NH4 + assimilation is energetically more favorable than NO3 −, it is usually toxic for plants. In order to study if an improved ammonium assimilatory metabolism could increase the plant tolerance to ammonium nutrition, tomato (Solanum lycopersicum L. cv P-73) plants were transformed with an NH4 +-dependent asparagine synthetase (AS-A) gene from Escherichia coli (asnA) under the control of a PCpea promoter (pea isolated constitutive promotor). Homozygous (Hom), azygous (Az) asnA and wild type (WT) plants were grown hydroponically for 6 weeks with normal Hoagland nutrition (NO3 −/NH4 + =6/0.5) and high ammonium nutrition (NO3 −/NH4 + =3.5/3). Under Hoagland''s conditions, Hom plants produced 40–50% less biomass than WT and Az plants. However, under NO3 −/NH4 + =3.5/3 the biomass of Hom was not affected while it was reduced by 40–70% in WT and Az plants compared to Hoagland, respectively. The Hom plants accumulated 1.5–4 times more asparagine, glycine, serine and soluble proteins and registered higher glutamine synthetase (GS) and glutamate synthase (GOGAT) activities in the light-adapted leaves than the other genotypes, but had similar NH4 + and NO3 − levels in all conditions. In the dark-adapted leaves, a protein catabolism occurred in the Hom plants with a concomitant 25–40% increase in organic acid concentration, while asparagine accumulation registered the highest values. The aforementioned processes might be responsible for a positive energetic balance as regards the futile cycle of the transgenic protein synthesis and catabolism. This explains growth penalty under standard nutrition and growth stability under NO3 −/NH4 + =3.5/3, respectively. [Copyright &y& Elsevier]