The gene encoding a rice endosperm-specific ADPglucose pyrophosphorylase (AGPP) subunit was isolated and its structure determined by nucleotide (nt) sequencing. A comparison of the genomic and cDNA nt sequences revealed a complex gene structure with ten exons and nine introns spanning over 6 kb. The exons ranged in size from 293 to 99 nt and the introns were between 1435 and 84 nt in size, with the first intron being the largest. All of the intron splice sites, except intron-2, contained GT/AG borders and were similar to the published splice site consensus sequences. Intron-2 had CA/CC borders at the 5' and 3' ends, but sequences adjacent to the splice site borders shared homology to the splice site consensus sequence, suggesting that the overall splice region, rather than the specific GT/AG sequence, determines the splice site. Several sequence motifs which may play a role in the regulation of plant genes were evident upstream from the transcriptional start point. Analysis of the developmental pattern of expression revealed a maximum level of mRNA transcripts for AGPP at five days after flowering coincident with starch accumulation. This result suggests that starch biosynthesis is controlled at the transcriptional level during seed development.