Serum levels of luteinizing hormone (LH) and prolactin were measured twice daily, and those of progesterone once daily, throughout gestation in rats with confirmed pregnancies, and in which parturition was seen to have occurred on day 23. Serum LH concentration during the first 11 days of pregnancy was distinctly higher than during the period between days 13–19, with a conspicuous pivot—point occurring at about day 12; beginning on day 20 and continuing to term, a progressive increase occurred, which was not contiguous with the acute, post—partum, ovulation inducing surge of LH. Serum prolactin level did not show as distinct a pattern of change, although highly erratic levels during the initial 10 days, and consistently low levels during most of the second half of pregnancy were noted. When related to the pattern of changes in the levels of serum LH and prolactin, the changes in serum progesterone level were seen to be inversely related to those in serum LH during the period from day 11 to term (r = -0.971). (Endocrinology92: 1527, 1973)