Soybean stems were grafted between the first and second nodes 6 weeks after planting. Three or 5 weeks after grafting, one root system was cut from grafted plants, doubling the shoot: root ratio. This technique was applied to plants grown in sand culture and supplied with an excess of water and minerals but with no combined nitrogen so that doubling the shoot:root ratio greatly increased the requirement for fixed nitrogen. When the shoot:root ratio was doubled during the flowering or seed formation stages, there was no statistically significant effect on total nitrogen or dry matter of shoots compared to non-grafted controls at maturity. The period between 2 and 21 d after doubling the shoot:root ratio was studied to determine changes in root weight, nodule weight, and rate of nitrogen fixation (acetylene reduction). Weight of roots and nodules increased relative to controls (grafted plants with one shoot per root) after about 1 week. The rate of acetylene reduction per gram of nodule was 75 per cent greater with roots having two shoots than with control roots 2 d after doubling the shoot:root ratio. Acetylene reduction per nodule declined to near control rates as nodule weight increased, but roots with two shoots maintained a 60 to 70 per cent greater rate of acetylene reduction per root for 2 weeks. The results indicated that nodulated soybean roots are capable of fixing nitrogen at rates greater than those which normally prevail.