SUMMARY Total hysterectomy of the guinea-pig before the 14th day of the oestrous cycle allowed the maintenance of the corpora lutea whereas if this procedure was performed on or after the 15th day the corpora lutea usually regressed at the normal time. The luteolytic action of the uterus in this species therefore takes effect during the 14th and 15th days after oestrus. Selective severing of the connexions between the uterus and the ovary suggested that this luteolytic effect is mediated via the main utero-ovarian blood vessels at least in the tubal region although a lymphatic pathway has not been eliminated.