The aim of this work to study the relationship between the activity of the enzymes phosphoenolpyruvate carboxilase (PEPcase) and glutamine synthetase (GS) with biological nitrogen fixation, in nodules, during the onthogenesis of bean (Phaseolus vulgaris L.) and Cowpea (Vigna unguiculata walp L.) plants. A greenhouse experiment was carried out using the symbiosis Rhizobium leguminosarum bv. phaseoli (CIAT-899)/Negro Argel and Bradyrhizobium sp. (CB-756)/Cowpea symbiosis. Plants were grown on a 1:1 silica/vermiculite·substrate and were watered with a nutrient solution without mineral-N. Ten samples of plants were take during the·growth cycle of both leguminous plants. The parameters analyzed in both cases were: activity of the enzymes PEPcase, GS and nitrogenase; number and fresh weight of nodules; nitrogen content and dry weight of roots, stalk+leaves and pods. In general, the activities of the enzymes PEPcase·and GS were positively correlated with that of nitrogenase and nitrogen fixation, as nodule fresh weigth and planto nitrogen content. In Negro Argel, the activities of PEPcase, GS and nitrogenase apresented two peaks during its entrie onthogenesis, one just before flowering and another during the mid pod filling stage. Cowpea, however, presented on1y one peak during "pre-flowering". The data reported in this experiment shows that the of N₂ fixation process depends not only on also on the reduction of N₂ (nitrogenase enzyme), but also on the assimilation of envolved carbon (PEPcase) and nitrogen which had been fixed (GS).