Abstract.-RNA prepared from the seminal vesicles of intact adult rats, or of testosterone-treated castrate adult rats, produced a 30 per cent increase in the weight of the seminal vesicle and a 25 per cent increase in the incorporation of labeled amino acids into protein. RNA prepared from the liver or ventral prostate of the same animals produced only a 10 per cent increase in the weight of the seminal vesicle. RNA prepared from the liver of control or testosteronetreated adult female rats was ineffective in stimulating the growth of the seminal vesicle. Polyuridylic acid instilled into the lumen had no effect on the weight of the seminal vesicle, but increased the incorporation of labeled phenylalanine into protein. RNA sedimenting in the 18S area on sucrose density gradients was most effective in stimulating growth of the seminal vesicle of three-week-old rats. Treating homogenized tissue with deoxycholate greatly increased the effectiveness of the RNA extracted. RNA extracted from seminal vesicles by treatment with deoxycholate and dextran sulfate was biologically active, whereas the usual 4S RNA in the 100,000 X g supernatant is ineffective.