Thymidine kinase activity of the ventral prostate from adult rats could be stimulated by administration to the animal of casomorphin or casomorphin (morphiceptin). On the other hand addition of both drugs to the culture medium of prostatic cells from immature rats enhanced the cells growth. In both cases, morphiceptin was more efficient than casomorphin. These results suggest than casomorphins have other properties than their morphinomimetic effects previously described.