The effect of 5α-dihydrotestosterone on RNA synthesis in the rat ventral prostate has been studied in vitro. The steroid was added to the incubation medium in a concentration of 10−6 m. Experiments have been performed with slices, homogenates, crude nuclear preparations and isolated nuclei from the rat ventral prostate. The results appear to indicate that there is no immediate quantitative effect of the steroid on RNA synthesis when administered in vitro.