Soybean plants (cv. Harosoy 63) inoculated with rhizobia were germinated from seed and beginning on day 7 after planting were subjected to one of four patterns of nutrient fertilization. One group received a nutrient solution containing 5 mM nitrate, a second group received nitrate-free nutrient solution, and two other groups received nitrate-containing solution either from days 7 to 13 or from days 14 to 20. On day 21 rates of leaf photosynthetic CO2 uptake and nitrate reductase activity were measured, as well as the capacities of the leaf to export recent photosynthate and of the nodules to reduce acetylene. The data support the hypothesis that sufficient nitrate availability in the leaves of young soybean plants can modify both photosynthetic CO2 uptake and light-mediated photoassimilate export to an extent that nodule development and the capacity for nitrogen fixation are reduced.