We screened steroid derivatives and found that spironolactone, an inhibitor of both 17beta-hydroxysteroid dehydrogenase (17beta-HSD) and aldosterone receptor, is an inhibitor of phytohormone brassinosteroid (BR) action in plants. Under both dark and light growing conditions, spironolactone induced morphological changes in Arabidopsis, characteristic of brassinosteroid-deficient mutants. Spironolactone-treated plants were also nearly restored to the wild-type phenotype by treatment with additional BRs. In the spironolactone-treated Arabidopsis, the CPD gene in the BR biosynthesis pathway was up-regulated, probably due to feedback regulation caused by BR-deficiency. Spironolactone-treated tobacco plants grown in the dark showed expression of light-regulated genes as was observed in the deficient mutant. These data suggest that spironolactone inhibits brassinosteroid action probably due to the blockage of biosynthesis and exerts its activity against plants. Thus, spironolactone, in conjunction with brassinosteroid-deficient mutants, can be used to clarify the function of BRs in plants and characterize mutants. The spironolactone action site was also investigated by feeding BR biosynthesis intermediates to Arabidopsis grown in the dark, and the results are discussed.