Total estrogen, luteinizing hormone (LH), and progesterone concentrations were measured in plasma samples collected on days −6, −5, −4, −3, −2, −1 before estrus, estrus (day 0) and days 3, 6, 12, 18 and 24 of pregnancy in six gilts. The mean progesterone concentration decreased from 16.1 ng/ml of plasma (per pig) on day −6 before estrus to 1 ng/ml or less on day −4. Progesterone levels remained low through estrus, increased to a peak value of 35.4 ng/ml on day 12 of pregnancy, and then declined to 17.2 ng/ml on day 24. Total estrogen began to increase on day −3, which coincided with the time of lowest progesterone levels in most animals, and reached maximum values of about 38 pg/ml on days −2 and −1 before estrus. Prior to the proestrous peak and during pregnancy, levels of estrogen fluctuated between 10 and 28 pg/ml. There was a significant negative linear correlation (r = −0.369, n = 65) between ng of progesterone and pg of estrogen (p < 0.01) during the experimental period. LH values were the following: days −6 through −2 (1.02 ng/ml) , days −1 and 0 (3.12 ng/ml) and days 3 through 24 of pregnancy 1.61 ng/ml) . The LH concentration was significantly greater during pregnancy than during the early follicular phase (p < 0.05). These results suggest that in the pig: 1) ovarian estrogen secretion during proestrus is inhibited until the progesterone secretion has fallen to an insignificant level, and the significant rise in estrogen levels that follows triggers the release of LH at the onset of estrus, and 2) the CL of pregnancy are dependent upon continued secretion of LH after estrus. (Endocrinology91: 675, 1972)