Retinoic acids (RAs) exert pleiotrophic effects on cellular growth, differentiation, and testicular functions. The primary function of Leydig cells is to produce testosterone whose intratesticular level is critical for spermatogenesis. Steroidogenic Acute Regulatory (StAR) protein is involved in transporting cholesterol from outer mitochondrial membrane to matrix side of inner mitochondrial membrane where cholesterol side chain cleavage (SCC) enzyme reside. Thus, the StAR protein is essential for acute response of steroidogenesis in steroidogenic tissues and the action of the protein is thought to be rate-limiting step for the testosterone synthesis. In order to understand how RAs control steroidogenesis in Leydig cells, cultured mouse Leydig tumor cells K-28 were treated with RAS and the mRNA levels of StAR gene were monitored by Northern blot analysis. Our study showed that RAs up-regulate the StAR mRNA in K-28 cells in time- and dose-dependent manner while the levels of the SCC mRNA were unchanged. The amount of progesterone produced from RA treated K-28 cells was increased concomitantly with the level of StAR mRNA, which indicates that StAR protein is the key regulator for steroidogenesis.