The time of synthesis, the molecular weight, and the relative glutamine-glutamate and proline to leucine ratios of the endosperm proteins of wheat (Triticum aestivum L. cv. Logan) were determined using a sodium dodecyl sulfatepolyacrylamide gel technique. In general, synthesis of most proteins occurred through much of the maturation of the seed, but past 20 days the rate of synthesis of the high molecular weight proteins declined more rapidly than those of lower molecular weight. The labeled "C amino acid mixture (0.1 mCi/0.1 mg/ml in proportions typical of an algal protein hydrolysate, Volk Radio Chemical Co., Burbank, Calif.) for 20 hr at 25 i 5 C. The final pH of the incubation mixture (including seed) was between 5 and 5.5. Subsequent experiments using buffered "C-labeled amino acid solutions have yielded similar results. Incubation was done in 5-mm i.d. test tubes which held the seed upright allowing the seed's vascular system to be submerged in the 30-,l droplet. Seeds used for determining the glutamate, glutamine, or proline to leucine ratios were harvested at 14 days and treated as above except that the appropriate mixtures of uniformly labeled 14C glutamate, 14C glutamine, and 14C proline with nominally labeled 4,5-3H leucine in pH 4, 0.01 M P04 buffer were used as incubation mixtures (0.0176 mCi 14C, 0.083 mCi 3H per ml; New England Nuclear, Boston, Mass.). After incubation, the seeds were rinsed with distilled H2O; the endosperms were dissected from the seed and frozen at -80 C until the time of extraction. As seed maturation proceeded, the amount of label incorporated relative to the protein present in the seeds steadily decreased until at 26 days the gels had to be heavily loaded in order to get sufficient radioactivity for counting individual bands.Proteins from mascerated endosperm tissue were solubilized by incubating the tissue in a solution containing 8 M urea, 2% (w/v) SDS (Pierce Chemical, Rockville, Md.), 5% (w/v) mercaptoethanol, and 0.0625 M tris, pH 6.8, for 48 hr at 25 i 5 C with periodic stirring. Throughout the remainder of this paper the word protein will include polypeptides released by disulfide bond reduction as well as proteins not so derived. The proteins were stored up to 10 days in the solubilizing solution. The discontinuous buffer system of Laemmli (4)