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.393607611