Anadenanthera colubrina (angico branco) e Anadenanthera peregrina (angico vermelho) are legume trees that in symbiosis with Rhizobium produce root nodules. We investigated the effect of two different sources of combined nitrogen on the initial growth and nodulation of these two species. The control plants received nutrient solution without addition of mineral nitrogen, and the others received nutrient solution containing either KNO3 (10 or 20mg N.plant-1.week-1) or (NH4)2 SO4 (10 or 20mg N. plant-1. week-1). The plants were harvested at 30, 60 and 90 days after sowing. Plants of A. colubrina, presented an increase in shoot dry mass when they received both combined nitrogen forms, mainly at the highest concentration. The two ammonium concentrations and the lowest nitrate concentration increased A. peregrina shoot dry mass, whereas the highest nitrate level did not affect plants dr y mass. The nodule root formation varied in the three harvests of the different treatments. At 90 days, nodules were observed in all treatments in A. colubrina plants, but no nodule formation was observed in A. peregrina plants fed with nitrate.393607611Becana, M., Sprent, J.I., Nitrogen fixation and nitrate reduction in the root nodules of legumes (1987) Physiol.Plantarum, 70, pp. 757-765Becana, M., Minchin, F.R., Sprent, J.I., Short-term inihibition of legume N2 fixation by nitrate (1989) Planta, 180, pp. 40-45Becker, M., Diekmann, K.H., Ladha, J.K., De Datta, S.K., Ottow, J.C.G., Effect of NPK on growth and nitrogen fixation of Sesbania rostata as a green manure for lowland rice (Oryza sativa L.) (1991) Plant Soil, 132, pp. 149-158Eaglesham, A.R.J., Nitrate inhibition of root-nodule symbiosis in doubly rooted soybean plants (1989) Crop Sci., 29, pp. 115-119Faria, S.M., Hay, G.T., Sprent, J.L., Entry of rhizobia into roots as Mimosa scabrela Benthan occurs between epidermal cells (1988) J.Gen.Microbiol., 134, pp. 2291-2296Hungria, M., Barradas, C.A.A., Wallsgrove, R.M., Nitrogen fixation, assimilation and transport during the initial growth stage of Phaseolus vulgaris L (1991) J. Exp. Bot., 42, pp. 839-844Keltjens, W.G., Van Loenen, E., Effects of aluminium and mineral nutrition on growth and chemical composition of hydroponically grown seedlings of five different forest tree species (1989) Plant Soil, 119, pp. 39-50Kwon, D.K., Beevers, H., Adverse effects of nitrate on stem nodules of Sesbania rostrata Brem (1993) New Phytol., 125, pp. 345-350Ligero, F., Caba, J.M., Lluch, C., Olivares, J., Nitrate inhibition of nodulation can be overcome by the ethylene inhibitor aminoethoxyvinylglycine (1991) Plant Physiol., 97, pp. 1221-1225Manhart, J.R., Wong, P.P., Nitrate effect on nitrogen fixation (acetylene reduction) (1980) Plant Physiol., 65, pp. 502-505McElhannon, W.S., Mills, H.A., The influence of N concentration on NO3/NH+4 ratio of the growth of lima and snap bean and southern field pea seedlings (1977) Commun. Soil Sci. Plant Anal., 8, pp. 667-687Nelson, L.M., Variation in ability of Rhizobium leguminosarum isolates to fix dinitrogen symbiotically in the presence of ammonium nitrate (1983) Can. J. Microb., 29, pp. 1626-1633Pate, J.S., Dart, P.J., Nodulation studies in legumes IV. The influence of inoculum strain and time of application of ammonium nitrate on symbiotic response (1961) Plant Soil, 15, pp. 329-346Schiavinato, M.A., Sprent, J.I., Influência da forma de nitrogênio combinado no crescimento inicial e na nodulação de Anadenanthera falcata (1993) IV Congresso Brasileiro de Fisiologia Vegetal., p. 70. , ResumoSnedecor, G.W., Cochran, W.G., (1967) Statistical Methods, , Univ. Press. Ame Iowa StateStreeter, J.G., Effect of nitrate in the rooting medium on carbohydrate composition of soybean nodules (1981) Plant Physiol., 68, pp. 840-844Streeter, J.G., Synthesis and accumulation of nitrite in soybean nodules supplied with nitrate (1982) Plant Physiol., 69, pp. 1429-1434Umali-Garcia, M., Libuit, J.S., Baggayan, R.L., Effects of Rhizobium inoculation on growth and nodulation of Albizia falcataria (L) Fosh. and Acacia mangium Willd. in nursery (1988) Plant Soil, 108, pp. 71-78Volk, R., Chaillou, S., Mariotti, A., Morot-Gaudry, J.F., Beneficial effects of concurrent ammonium and nitrate nutrition on the growth of Phaseolus vulgaris: A 15N study (1992) Plant Physiol. Biochem., 30, pp. 487-493