Progesterone is an animal steroid hormone that functions as a corpus luteum hormone which maintains pregnancy and implantation of fertilized egg. Recently, we identified progesterone from Arabidopsis, rice, tomato and pea, suggesting that progesterone is a ubiquitous constituent of plants. To clarify the biological function of progesterone in plants, Arabidopsis(Columbia) was germinated and grown in the presence or absence of progesterone under the normal or weak light(13μmol/m^2/s) or in the dark. Under the weak light, the growth of Arabidopsis hypocotyl was promoted by 10nM-1μM progesterone, but inhibited by 100μM progesterone. In the dark, progesterone did not affect the Arabidopsis hypocotyl elongation. After the growth for 30 days under the weak light, Arabidopsis petiole was elongated to 130% that of control. In the light, rossette leaves of Arabidopsis plants grown in the presence of progesterone and an ethylene biosynthesis inhibitor AVG were expanded larger than those grown with only AVG. These results indicate that progesterone may exhibit acts growth promoting activity on plants. Studies on the effects of progesterone on root development, gravitropism and flowering timing are now in progress.