The size of plants is largely determined by growth of the stem. Stem elongation is stimulated by gibberellic acid(1-3). Here we show that internode stem elongation in rice is regulated antagonistically by an 'accelerator' and a 'decelerator' in concert with gibberellic acid. Expression of a gene we nameACCELERATOR OF INTERNODE ELONGATION 1(ACE1), which encodes a protein of unknown function, confers cells of the intercalary meristematic region with the competence for cell division, leading to internode elongation in the presence of gibberellic acid. By contrast, upregulation ofDECELERATOR OF INTERNODE ELONGATION 1(DEC1), which encodes a zinc-finger transcription factor, suppresses internode elongation, whereas downregulation ofDEC1allows internode elongation. We also show that the mechanism of internode elongation that is mediated byACE1andDEC1is conserved in the Gramineae family. Furthermore, an analysis of genetic diversity suggests that mutations inACE1andDEC1have historically contributed to the selection of shorter plants in domesticated populations of rice to increase their resistance to lodging, and of taller plants in wild species of rice for adaptation to growth in deep water. Our identification of these antagonistic regulatory factors enhances our understanding of the gibberellic acid response as an additional mechanism that regulates internode elongation and environmental fitness, beyond biosynthesis and gibberellic acid signal transduction.