During maize endosperm development, cells shift from a mitotic cycle to endoreduplication, driving the massive synthesis of storage proteins (zeins) and starch. In this developmental context, we studied changes in expression levels of histone H1 and high mobility group I/Y (HMG-I/Y), two chromatin architectural proteins that are known to affect gene transcription. Almost no change was found in the level of histone H1 during endosperm development, despite a dramatic increase in DNA content (endoreduplication); hence, the histone H1/DNA ratio decreased substantially. Concurrently with a reduction in the Cdc2 kinase activity at the shift to endoreduplication, significant changes were found in the level and mobility of the HMG-I/Y protein; the faster migrating forms were, at least partly, hypophosphorylated. Purified maize HMG-I/Y protein was found to be phophorylated in vitro by the Cdc2 kinase and bound efficiently to the γ-zein promoter AT-rich tract (γZ-AT). Using an in vitro transcription assay, we demonstrated the capability of the maize HMG-I/Y protein to relieve the inhibitory effect exerted by histone H1 on templates containing the γZ-AT sequence. These data suggest that during maize endosperm development transcription and perhaps replication are controlled, at least partly, by the activity of the Cdc2 kinase and the interplay between histone H1 and HMG-I/Y proteins.