The effects of the aluminum, nitrate and ammonium interaction, in nutritive solution, were studied under controlled conditions. An analysis was performed on the growth, potassium concentrations, and percentage composition of N of the amino acids in Stylosanthes guianensis and S. macrocephala, sensitive and resistant, respectively, both to aluminum and to ammonium. The ammonium nutrition reduced the growth of the S. guianensis drastically. The ammonium toxicity, verified only in the S. guianensis, was associated to the stimulation of the metabolic routes syntheses of proline and arginine, starting from the glutamic acid, to the detriment of the glutamine synthesis. The aluminum partially alleviated the ammonium toxicity in the S. guianensis, increasing the asparagine retention and reducing the arginine composition in the root system. In the presence of the nitric source, the aluminum increased the potassium concentrations only in the S. macrocephala. In the presence of nitric source, the interconversion of N-aminic was influenced by aluminum in both species, however the percentage composition of N-amino acids was more modified in the S. macrocephala.