The time course of ADP-glucose pyrophosphorylase activity and of starch accumulation rate measured in grain, from pollination to maturity, in Zea mays L. plants grown outdoors, was coincident for 2 years. No such correlation was observed in the adjacent leaf, which, furthermore, presented large year-to-year differences in starch accumulation pattern. Analysis of the expression of ADP-glucose synthase at the protein levels, using antibodies directed against the Bt2 or Sh2 subunits, established that the variation of activity in the grain was explained by parallel changes in the content of both subunits. The cDNA for Bt2 and Sh2 subunits were used as probes to quantify the corresponding messenger. In grain, the time course of Bt2 and Sh2 mRNA accumulation anticipated, with a similar pattern, the specific peptide variations, which suggests a transcriptional control of expression. By contrast, the control of leaf activity by protein content was less obvious than in the grain, and changes in leaf enzyme specific activity were suggested during the first 20 d after pollination. A clone homologous to the grain Bt2 subunit cDNA was isolated from a maize leaf cDNA library, and a sequence comparison showed that the leaf clone (L2) was a partial cDNA representing one-third of the mature peptide. A 97% homology was observed between Bt2 and L2 in their coding region, but homology was poor in the 3' noncoding border. This result demonstrates that Bt2 and L2 arise from different genes presenting a tissue-specific expression pattern and provides an explanation for the earlier reported differences between leaf and grain in the size of peptide and mRNA for the Bt2-homologous subunit.