The role of gibberellins (GAs) in the control of growth and flowering in stock, Matthiola incana (L.) R. Br., and the possibility of their regulation by manipulation of GA biosynthesis were investigated. In M. incana, GAs were found to be necessary for stem elongation and flowering. GA₄ in the non-13-hydroxylation pathway of GA biosynthesis was the most active component for stem elongation and flowering. It was shown that GA biosynthesis inhibitors of acylcyclohexanedione type could promote stem elongation and flowering. These results indicate that the balance of activities among GA biosynthetic and catabolic enzymes is very important for stem elongation and flowering. Furthermore, development of responsiveness to active GAs for flowering may be one of the essential factors in the flower bud initiation process in M. incana. Based on the above results, we demonstrated that acceleration of the flowering time in summer-sown M. incana could be obtained by prohexadione-calcium (PCa) treatment under plastic-film greenhouse conditions.