The subcutaneous implantation of compressed tablets of pure crystalline material has proved a highly successful method of administering steroid hormones. The essential feature of the technique, which was introduced by Deanesly & Parkes [1937], and which has given particularly favourable results with oestrone, oestradiol, progesterone, testosterone and desoxycorticosterone, is that it allows a very prolonged action for periods up to years from a single administration, and a very high effectiveness in relation to the total dose. The fact that months or years may be required for the complete absorption of a tablet depends on the low solubility of the steroid substances in body fluids, and it would be expected that the existing technique could be applied only to substances of this kind. However, attempts have recently been made to adapt the implantation technique to the administration of a variety of other substances, some of them having a much greater solubility