Agropastoral systems that combine soybean and grasses in a crop rotation have been proposed for sustaining grassland productivity in the low-fertility soils of the Brazilian savanna. We studied the nitrogen flow under an agro-pastoral system developed in 1993 in a Purple Red Latosol in Campo Grande, MS. Four cropping systems were included: continuous soybean cropping, soybean cropping after 4 years of grass cultivation, continuous grass cultivation, grass cultivation after 4 years of soybean cropping. The rate of N fixation in soybean was determined by the N difference method and 15N natural abundance method. A non-nodulating isoline, T201, was used as a control in both methods. The rate of N2 fixation ranged from 23 to 51 % of the total plant N. The amount of N taken out as grains was larger than the amount of fixed N in soybean. The amount of N entering the system through rainwater was 6 kgN/ha/yr. Nitrate accumulating in soil from the surface to a depth of 100 cm under soybeans indicated the potential leaching of nitrate. The estimated amount of nitrate leaching from the soybean fields was large compared with that from the grasslands. A large negative N balance of 134-211 kgN/ha/yr was estimated in the soybean fields, whereas the N balance in the grasslands was a slightly negative.