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Short periods of treatment with progesterone precipitate uterine bleeding in a spayed monkey which is receiving constant daily doses of oestrogen. The amount of progesterone required is proportional to the dose of oestrogen given.

Treatment with progesterone (5 mg daily for 5 days) also precipitates uterine bleeding in untreated spayed monkeys at regular intervals of about 8 days. The first course of injections must be started immediately a menstrual bleeding has been induced, either by the surgical removal of the ovaries or by the withdrawal of an adequate oestrogenic stimulus. Treatment is usually ineffective if it is delayed for as little as 6 days.

The number of cycles obtained in spayed animals, pretreated with oestrogen, seems to be related to the size of the initial dose of oestrogen used to induce withdrawal bleeding.

Such cycles can be obtained not only after an oestrogen-withdrawal bleeding but also after a progesterone-withdrawal bleeding has occurred in monkeys which have neither received oestrogen nor menstruated for several months before treatment with progesterone was begun. Conditioning of the subsequent responses of the uterus for many months to come depends, therefore, on the occurrence of a single menstrual bleeding but not on the particular hormone used to obtain the bleeding.

These effects of progesterone in spayed monkeys cannot be simulated by injections of deoxycorticosterone acetate (DCA).

The intermenstrual bleeding that can be induced in normal female rhesus monkeys by injecting 5 mg progesterone daily for 5 days does not occur if the injections are continued at a constant lower threshold dose, which varies for different monkeys between 0·25 and 1·0 mg daily.