Aspartate, alanine and glutamate uptake into bacteroids and symbiosomes isolated from soybean nodules were measured. An aspartate transport mechanism was identified in free bacteroids, with an apparent Km of 27.5 micromolar and a Vmax of 5.14 nmol mg-1 min-1. Inhibitor studies indicated that aspartate uptake was dependent on a proton motive force across the bacteroid membrane. Aspartate transport was competitively inhibited by glutamate with an apparent Ki of 9.7 micromolar. Alanine uptake into bacteroids also showed saturation kinetics with a Km of 12.2 micromolar and Vmax of 3.5 nmol mg-1 min-1. Bacteroids loaded with 14Caspartate exchanged label for external cold aspartate, and glutamate. In contrast, there was no evidence for aspartate or alanine movement into or out of intact symbiosomes except by slow passive diffusion.