THE CONTRACEPTIVE ACTION OF IMPEDING OESTROGENS VI. CONTRACEPTIVE ACTIVITY OF OESTRIOL DERIVATIVES

post coitum (p.c.) resulted in prevention of implantation of blastocysts at doses of 0\m=.\25mg/day. Similarly, an analogue of oestriol, 18-homo-oestriol, prevented conception in oral doses of 1 mg or more. It is reasonable to consider that the dose of steroid required for prevention of implantation may depend on the rate of its metabolic removal and on its solubility in adipose tissue for storage, among other factors. Thus, triesters of oestriol, which can be stored more readily by virtue of their greater solubility, i.e. lipophylic character, and are also capable of being hydrolysed in vivo, might exhibit activity at lower concentrations. Four groups each of four Sprague-Dawley rats (250 g) were mated during pro-oestrus. Mating was checked by observation of spermatozoa in the vaginal smear. Following mating, the animals were anaesthesized with ether and given an oral infusion of (a) 1 ml of sesame oil, (b) 0-15 mg of oestriol (0-52 µ ) in sesame oil, (c) 0-263 mg of oestriol tripropionate (0-52 µ ) in sesame oil, and(d) 0-216 mg of oestriol triacetate (0-52 µ ) in sesame oil, daily for 4 days. On the 10th day following proven mating, the animals were killed, the uteri removed and the number of implantation sites recorded