Maize endosperm is dependent on source tissues to supply the energy and carbon required for development. This supply varies during the course of each day and also throughout development. The impact of these variations on the metabolism of developing endosperm was examined by determining the energy status of the endosperm throughout the course of a day. The adenylate energy charge decreased as the tissue matured, and exhibited a distinct diurnal pattern, reaching a minimum in the afternoon, when the flux of photosynthate is the highest. The minimum value observed was similar to the adenylate energy charge in tissues under mild stress. As the endosperm matured, the adenylate energy charge decreased steadily. The levels of the polysaccharide precursors ADP-glucose and UDP-glucose did not reflect the daily fluctuations in adenylate energy charge, but did exhibit similar long-term behaviour in the latter half of development, decreasing steadily after 21 d after pollination. Similarities in the metabolic patterns of adenylate and uridylate nucleotide levels are discussed in terms of the analogous roles of these compounds in starch and cellulose biosynthesis, respectively. These data provide insight into the metabolic rhythms occurring during endosperm development, and provide a framework for efforts directed toward metabolic engineering.