We examined the effects of brassinosteroids on Arabidopsis thaliana (L.) Henyh. ecotype Columbia in order to develop a model system for studying gene regulation by plant steroids. Submicromolar concentrations of two brassinosteroids, brassinolide and 24-epibrassinolide, stimulated elongation of Arabidopsis peduncles and inhibited root elongation, respectively. Furthermore, brassinolide altered the abundance of specific in vitro translatable mRNAs from peduncles and whole plants of Arabidopsis. Root elongation in the auxininsensitive Arabidopsis mutant axrl was inhibited by 24-epibrassinolide but not by 2,4-D, indicating an independent mode of action for these growth regulators in this physiological response.Brassinosteroids are plant growth-promoting natural products with structural similarities to insect and animal steroid hormones (Mandava 1988). Brassinolide (BR), the first brassinosteroid to be structurally characterized (Grove et al. 1979), was originally isolated from Brassica napus pollen and was shown to induce cell elongation and proliferation at nanomolar concentrations in bean, rice, and many other plants. Subsequent work identified other brassinosteroids with similar structures from a broad range of plants including rice, maize, mustard, and wheat (Mandava 1988). The physiological effects and agricultural applications of brassinosteroids have been widely studied, and there is growing evidence that these compounds have properties characteris-