The hypothesis that soybean (Glycine max L. [Merrill]) catabolizes ureides to urea to a physiologically significant extent was tested and rejected. Urease-negative (eu3-e1/eu3-e1) plants were supported by fixed N2 or by 2 mM NH4NO3, so that xylem-borne nitrogen contained predominantly ureides (allantoin and allantoic acid) or amide amino acids, respectively. Seed nitrogen yield was equal on either nitrogen regime, although 35-d-old fixing plants accumulated about 6 times more leaf urea. In callus, lack of an active urease reduced growth on either arginine or allantoin as the sole nitrogen source, but the reduction was greater on arginine (73%) than on allantoin (39%). Furthermore, urease-negative cells accumulated 17 times more urea than urease-positive cells on arginine; for allantoin the ratio was 1.8. Urease-negative callus accumulated urea at 3% the rate of seedlings. To test whether urea accumulating in urease-negative seedlings was derived from ureides, seeds were first allowed to imbibe in 1 mM allopurinol, an inhibitor of ureide formation. Seedling ureides were decreased by 90%, but urea levels were unchanged. Thus, ureides are poor precursors of urea, which was confirmed in seedlings that converted no more than 5% of seed-absorbed [14C-ureido]allantoate to [14C]urea, whereas 40 to 70% of [14C-guanido]arginine was recovered as [14C]urea.