The N form had a pronounced effect on the levels of free amino acids. When compared to NO3, NH4 brought about a several fold increase in free amino acids (dry basis) in the roots, while smaller increases were observed in mature, fully expanded leaves, in young growing leaves, and in tomato fruit. Plants fed a combination of the 2 N forms had intermediate levels. The pattern of response to NH4 indicated that aspartic and glutamic acids as well as their amides were predominantly snythesized in the roots when the N source was NH4. The site of synthesis shifted to the leaves when the N source was NO3. The amino acids arginine, serine, alanine, and γ-aminobutyric acid were predominantly synthesized in the leaves irrespective of N form. Topical application of Ca on fruit grown with NH4 resulted in amino acid levels in the fruit similar to those of NO3 fruit. Ammonium toxicity was found to be accompanied by large increases in the levels of GABA and serine in the leaves. It is suggested that ammonium toxicity is a manifestation of intracellular Ca deficiency.