1. The minimum amount of oestrogen, given as a single intramuscular injection, that is required to induce oestrogen withdrawal uterine bleeding ('threshold dose') has been estimated for eleven oestrogens in twenty-five spayed adolescent or mature rhesus monkeys (Macaca mulatta). 2. The threshold dose was always smaller when it was approached from supra-threshold dosage levels (falling series) than from sub-threshold levels (rising series). 3. The oestrogens tested can be arranged in the following decreasing order of potency: oestradiol dipropionate, stilboestrol dipropionate, oestradiol caprylate, oestradiol benzoate, oestradiol benz-butyrate, oestrone benzoate, stilboestrol, hexoestrol, oestradiol, oestrone, dienoestrol. 4. Esterification of the oestrogens increased the potency and duration of bleeding, but diminished the latent interval between the injection and the onset of bleeding. 5. Latency before bleeding was shorter, and duration of bleeding was longer the more potent the oestrogen. 6. The latent interval before bleeding was shorter for falling series of injections than for rising series of injections. 7. At doses above the threshold, latency before bleeding and duration of bleeding tended to become shorter.