A new method of hormone administration was tested in the human male. PDS (silastic) capsules filled with dry, milled steroid and implanted subcutaneously, release the hormone at a relatively constant rate over a longer period of time. A standard capsule of 20 mm length, a membrane thickness of ∼0.8 mm and a surface area of about 150 mm2 was used. Capsules filled with 22–23 mg ethinyl estradiol and implanted in patients with carcinoma of the prostate had a releasing factor of approximately 45 µg/24 h/capsule, and a therapeutic effect of about 17 months. Capsules filled with 21–23 mg testosterone and implanted in castrates, hypogonadal and subfertile men had a releasing factor of approximately 55 µg/24 h/capsule and a therapeutic effect of about 13 months. The clinical results of testosterone substitution in castrates and hypogonadal men were good as judged by clinical observation and regular determinations of plasma testosterone levels. There was a definite improvement of the sperm qualities of subfertile men with moderate forms of Leydig’s cell insufficiency. The observation period of the cases of prostatic carcinoma treated with ethinyl estradiol capsules still is too short for exact clinical evaluation, but the results seem to be encouraging.