Cell suspension cultures of Arabidopsis thaliana (L.) Heynh. have been used to study the effect of auxin (a-naphthaleneacetic acid), cytokinin (kinetin), and sucrose on cell cycle gene expression. Auxin, cytokinin, and/or sucrose were re-added to culture media depleted for all three mitogens and the effects on the transcript levels of key cell cycle genes were monitored. Different cell cycle genes responded differentially to the mitogen supply. The cyclin-dependent kinase (CDK)-encoding gene CDKA;1 and the A- and D-type cyclins were induced by sucrose, whereas CDKB1;1 and CYCB1;1 were induced most strongly by the combined administration of auxin and cytokinin. The transcript levels of CDK inhibitors were up-regulated upon the addition of Gamborg's B5 salts to depleted medium. This induction was counteracted by the presence of auxin, cytokinin, or sucrose, depending on the CDK inhibitor. The obtained results are discussed in regard to the current knowledge on mitogen effects on cell cycle regulation in Arabidopsis.