1. Experiments were carried out on isolated strips of uterine muscle to determine the extensibility of the myometrium at different stages of pregnancy and post-partum.2. Results showed that the extensibility increases significantly after mating and remains fairly constant from days 7 to 27 of pregnancy. Thereafter there is a further significant increase in extensibility which lasts until delivery. During the first 24 hr post-partum the extensibility decreases rapidly.3. In unilateral pregnancy there appeared to be no difference in extensibility between the two horns.4. The extensibility found in the myometrium of oestrous females could be reproduced in ovariectomized rabbits treated with oestrogen. When progesterone was injected in addition to oestrogen, the extensibility was significantly increased but not to the same extent as in mid-pregnancy.5. The extra increase in extensibility found at the end of pregnancy was not reproduced in ovariectomized rabbits after long-term injections of oestrogen and progesterone followed by additional relaxin or diminished progesterone therapy or relaxin and diminished progesterone therapy.6. It is suggested that the changes in extensibility of the myometrium associated with pregnancy are hormone-based, that the main hormone is progesterone and that perhaps equivalent changes can only be reproduced in the ovariectomized rabbit by critical doses of oestrogen and progesterone.