An analysis was made of protein complement and metabolism in juvenile (J), emerging (E), mature (M) and senescent (S) leaves of Lolium temulentum L. Amounts of total protein increased markedly from J to E and declined to low levels from M to S. This trend was also followed by the individual thylakoid membrane proteins LHCP-2, OEC-33, D1 and cyt f, visualised using Western blotting, and by a number of soluble polypeptides detected by protein staining on SDS gel electrophoresis. Some soluble proteins were markedly stage-specific in abundance. Polypeptides of Mr 65 × 10 3 and 19 × 10 3 were uniquely associated with stage J; E and M tissue yielded a distinctive high-Mr component; and the proteins of M and S tissue included a constituent of Mr 24 × 10 3 . RNA was isolated from leaf tissue and the products of cell-free translation were fractionated by electrophoresis. Differences between stages of development in the complement of polypeptides made in vitro indicated pronounced modulations in the profile of translatable messages. Amongst the stage-specific products were a polypeptide of Mr 87 × 10 3 at J and 100 × 10 3 at S. Several similarities, and points of contrast, were seen between the M to S transition of attached leaves and the senesence process in detached leaves senescing in darkness. These results are discussed in terms of the role of differential gene expression in the regulation of leaf development and senescence.