Rice (Oryza sativa L.) panicles were harvested at 2-day intervals from 4-20 days post anthesis (DPA) and labeled with [35S]sulfate. The amount of protein synthesis at each sampling date was estimated by determining the amount of exogenously applied [35S]sulfate incorporated into acid-precipitable material. The largest amount of incorporation occurred at 8 DPA and decreased after 10 DPA. When total seed extracts were analyzed by sodium dodecyl sulfate-polyacrylamine gel electrophoresis (SDS-PAGE), the 35-39 and 20 kilodalton (kd) groups of polypeptides appeared as early as 6 DPA. When these polypeptides were compared with those of globulin, prolamin and glutelin fractions sequentially extracted from rice flour, they had the same electrophoretic mobility as glutelin. The largest accumulation of polypeptides occurred between 8 and 10 DPA. When radioactive peptides from seeds labeled in vivo were visualized by fluorography, there was a prominent 56-kd polypeptide but no polypeptides co-migrating with glutelin were evident. A pulse-chase experiment indicated that the 56-kd polypeptide could be chased into the 35-39 and 20 kd groups. Translation in vitro suggests that the 56-kd polypeptide was preferentially synthesized by membrane-bound polysomes.