In order to understand the control mechanism of progestin secretion during lactation, progesterone (P) and 20α-hydroxypregn-4- en-3-one (20α-OH-P) were determined in ovarian venous blood collected from rats nursing 2 or 6 pups at various stages of lactation in controls, and rats given LH antiserum, LH and prolactin. Generally, P secretion rate increased gradually to reach a peak on Day 12 of lactation and fell sharply on Day 16. Secretion rate of 20α-OH-P decreased as that of P increased and rose sharply on Day 16. On Day 8, P secretion averaged 13.5 μg/30 min in rats nursing 6 pups and was significantly higher than that in rats nursing 2 pups (7.2 μg/30 min). In rats nursing 6 pups LH antiserum reduced P secretion on Day 8 to the level of 2 pup control group. No difference in P secretion was seen between control and LH antiserum groups on Day 16 when P secretion was extremely low (0.6 μg/30 min). LH antiserum also reduced 20α-OH-P secretion in the 6 pup group on Days 4, 8 and 16. Treatment with LH antiserum did not interfere with lactation judged by normal increases in body weight of pups. LH increased P secretion in 2 pup group on Day 8 and in 6 pup group on Day 16. Prolactin treatment did not result in an increase in P and 20α-OH-P secretions on Days 4 and 8, but it reduced significantly the secretion of 20α-OH-P on Day 16. Thus, it was shown that LH is also involved in progestin secretion, during lactation, when the secretory activity is elevated, and that over-all progestin secretion in lactating rats is a reflection of the amount of trophic hormones secreted by the pituitary in response to the strength of suckling stimulus. (Endocrinology88:1126,1971)