The purpose of these experiments was to study some of the factors controlling the timing of mating behavior on the day of proestrus of the rat estrous cycle. In unoperated rats exhibiting 4-day cycles, the first appearance of the lordosis response to a male came after the onset (1400 hr) of the critical period for LH release; the average time was at 1641 hr. In unoperated 5-day rats half had shown the first lordosis by 1400; not only was the mean time of onset significantly earlier but the variance was significantly greater in the 5-day rat. Sham ovariectomy advanced the time of lordosis if performed early enough in the day of proestrus in both types of cyclers; ovariectomy also advanced lordosis except when performed at 0600 hr in 4-day rats, when the procedure blocked or delayed mating. Adrenalectomy delayed lordosis in both types of cyclers until after 1800 hr; combined ovariectomy and adrenalectomy blocked lordosis from appearing at all in significant numbers of animals of both cycle lengths. The combined surgery performed at 0600, followed by progesterone (2.5 mg) at 0800, elicited lordosis in 5-day cyclers, but not 4-day. Sham ovariectomy at 0600 hr in rats of either cycle length also advanced the time of LH discharge for ovulation, as deduced from barbital blockade experiments. It is concluded that the adrenal glands, presumably via progestin secretion, participate in the normal timing of mating behavior in the intact rat. Moreover, stressful surgical intervention can superimpose disturbances in this timing, as well as in the timing of LH release. (Endocrinology88: 325, 1971)