Any contraceptive method acts by producing temporary sterility. Var iations in the time of spontaneous ovulation and variations in the length of the cycle render the date of ovulation relatively unpredictabl e. The safe period, therefore, varies. Hormonal sterilization deals with methods that render the woman sterile for the duration of at least 1 menstrual cycle. This anovulatory cycle is 1 method. No corpus luteum forms although bleeding occurs cyclically. Most cycles during lactation are sterile. Other women sometimes have anovulatory cycles. Prolactin injections have been shown to temporarily suppress ovarian cyclic activity in mice and rats. Prolactin may produce anovulatory cycles in women and thus provide a method of hormonal sterilization. 4 mg progesterone daily inhibits estrus. Studies with estrone show that it has a depressant effect on ovarian response to follicle stimulating hormone. Large doses of estradiol benzoate used to treat dysmenorrhea have altered menstrual rhythm. Injection of gonadotrophic antisera for prevention of ovulation is a possibility. Daily injections of estrone, begun on day of mating, usually have resulted in retention of ova in the fallopian tubes of mice and rabbits. All of these ova showed signs of degeneration by Day 4. Fertilized human ova may be tube locked by an excess of estrone or degeneration of the fertilized ovum may inhibit further development. The potentialities of hormonal sterilization are great. Extensive investigation for humans is warranted.