Mature seeds of Glycine max (L.) Merr. contain two major storage proteins, a glycosylated 7S protein (conglycinin) and a non-glycosylated 11S protein (glycinin). Accumulation of these proteins and their mRNAs during seed development in cv. Provar was studied by SDS polyacrylamide gel electrophoresis and by "Northern" (DNA-RNA) hybridization. The 11S acidic and basic subunits and the 7S α' and α subunits began to accumulate 18-20 d after pollination, shortly after the termination of cell division in developing cotyledons, whereas the 7S β and 11S A-4 subunits were not detected until one to two weeks later, during the maturation phase of development. Messenger RNAs for 7S and 11S proteins were first detected 14-18 d after pollination, several days before the accumulation of storage proteins. Extracts from embryonic axes contained reduced levels of the 7S α subunit, very little 11S protein, no detectable 7S β or 11S A-4 subunits, and an additional 7S subunit not found in cotyledons. Soybean axes and cotyledons therefore differ in their synthesis of seed storage proteins.