In monocotyledonous plants, the process of seed development involves the deposition of reserves in the starchy endosperm and development of the embryo and aleurone layer. The final stages of seed development are accompanied by an increase in desiccation tolerance and drying out of the mature seed. We have used two-dimensional gel electrophoresis for a time-resolved study of the changes in proteins that occur during seed development in barley (Hordeum vulgare). About 1,000 low-salt extractable protein spots could be resolved on the two-dimensional gels. Protein spots were divided into six categories according to the timing of appearance or disappearance during the 5-week period of comparison. Nineteen different proteins or protein fragments in 36 selected spots were identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry (MS) or nano-electrospray tandem MS/MS. Some proteins were present throughout development (for example, cytosolic malate dehydrogenase), whereas others were associated with the early grain filling (ascorbate peroxidase) or desiccation (Cor14b) stages. Most noticeably, the development process is characterized by an accumulation of low-M(r) alpha-amylase/trypsin inhibitors, serine protease inhibitors, and enzymes involved in protection against oxidative stress. We present examples of proteins not previously experimentally observed, differential extractability of thiol-bound proteins, and possible allele-specific spot variation. Our results both confirm and expand on knowledge gained from previous analyses of individual proteins involved in grain filling and maturation.