Continuous supply of nitrate in the nutrient solution for 40 d inhibited nodule formation and nitrogenase (acetylene reduction) activity more in mungbean (Vigna radiata L.) plants inoculated with hydrogen uptake negative (Hup-) Rhizobium strain M11 compared to plants inoculated with hydrogen uptake positive (Hup+) strain S24. Nitrate even at a low concentration of 1 mM inhibited nodule forsmation and nitrogenase activity (per plant) by 57 and 46% in plants inoculated with Hup- strain, and by 34 and 20% in plants inoculated with Hup+ strain, respectively. At 10 mM nitrate concentration, however, nodule formation was almost completely depressed. Addition of IAA (10-6 M) before inoculation alleviated but did not completely reversed nitrate (4 mM) inhibition on nodulation process and nitrogenase activity in plants inoculated with Hup- strain. However, in plants inoculated with Hup+ strain the same concentration of IAA not only reversed inhibitory effect of nitrate (4 mM) but further stimulated nodulation and per plant nitrogenase activity by 20 and 65%, respectively. In vivo activity of leaf NR increased as the concentration of nitrate increased in the nutrient solution. These observations suggest that diversion of photosynthates towards nitrate assimilation may be responsible for depressing nodule growth and function and, in this situation Hup system might. be supporting nitrogen fixation. Further, a role for IAA in nodule biogenesis has been indicated.