Using an in vivo assay at pH 7.5 showed that greenhouse grown, sand uncultured cotton (Gossypium hirsutum, L.) petioles had nitrate reduction (NR) rates of 0.114 and 0.350 µmol NO₂/g FW/h at cotyledon stage and five mature (well developed) leaves stage, respectively. These rates were similar to those in the blades. Beginning with branching stage (without visible squares on the fruiting branches), petioles had a rate of NR significantly higher than the blades at the 5% level (0.617 vs 0.514, µmol NO₂/g FW/h). At the early flowering stage, the rate of NR decreased, but the difference between the two was four fold in favor of petioles (0.477 vs 0.118 µmol NO₂/g FW/h). At the flowering 2 after cut out stage, the rate of NR in the petioles was still about 60% more than in the blades. Nitrate nitrogen (NO₃-N) content of petioles was 4 and 6 times more than the blades at the cotyledon stage and the five mature leaves stage, respectively, and was about 10 times higher than the blades thereafter. Yet, the efficiency of the use of NO₃-N or the ratio of NO₃-N to NO₃-N was almost always higher in the blades.