Treatment of young rats with 3 mg. testosterone propionate per wk. led to decrease of testes wt., inhibition of sperm maturation, and interstitial tissue damage. After treatment was discontinued, spermatogenesis was partially restored and some animals became fertile. Larger doses (30 mg. per wk.) resulted in testis wt. increase and stimulation of spermato-genesis, whether given from the 1st to the 30th day or from the 30th to the 60th day of life. Upon treatment with 30 mg. per day from the 1st to the 60th day testis wt. wag decreased and spermatogenesis inhibited, though to a less degree than with a daily dosage of 3 mg. Stimulation of permatogenesis was possible only within limits, as increased activity may be evoked in immature germinal epithelium but not an earlier maturation.