The duration, dose and temporal interrelationship of three hormonal parameters: estrous (priming) estrogen, progesterone and nidatory estrogen, are concerned in the timing and degree of endometrial sensitivity to an artificial intrauterine decidual stimulus. Following priming by estrous estrogen the uterus is maximally responsive to subsequent treatment with progesterone and nidatory estrogen for a period of about three days starting on the fourth day after the last priming injection. During this time the fine timing is determined by the duration of progesterone and nidatory estrogen. Neither hormone alone is able to induce sensitivity but in combination sensitivity is maximal in three days. This is followed by a period of insensitivity. Progesterone alone induces a partially prepared state which can be held for several days until sensitivity is completed by estrogen.